Server error "A non-numeric value encountered"

I’m trying to set up a local copy of the Gantry documentation website, which is realized with Grav, on my internal server.
As I have little experience with Grav, I am running into various problems for which I cannot find a solution.

The purpose is to have a test environment for the translation of the gantry documentation.

It is best to describe my approach first, maybe my error has already occurred there:

  1. Fork of the github repository created
  2. made some changes there
  3. Installation of Grav on the local server (it works)
  4. downloaded a zip file from my fork
  5. copy the unzipped zip file into the /user directory of the Grav installation

Now the following error occurs when I call my site:

server error
0 - An exception has been thrown during the rendering of a template ("A non-numeric value encountered")

In the log file I find the following entry:

[2020-09-08 08:27:44] grav.CRITICAL: An exception has been thrown during the rendering of a template ("A non-numeric value encountered"). - Trace: #0 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/Template.php(422): Twig\Template->displayWithErrorHandling(Array, Array) #1 /var/www/clients/client1/web16/web/grav/cache/twig/2d/2d7b256791e792fcddcc52cc49984a6f069d3c0690f52bdce6e9056d2a283f6d.php(224): Twig\Template->display(Array) #2 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/Template.php(216): __TwigTemplate_c5943fef62a01b10b3a47fe42b774c67a6a2d78a6b0ba5672f86b0ff6a0ee652->block_sidebar(Array, Array) #3 /var/www/clients/client1/web16/web/grav/cache/twig/2d/2d7b256791e792fcddcc52cc49984a6f069d3c0690f52bdce6e9056d2a283f6d.php(57): Twig\Template->displayBlock('sidebar', Array, Array) #4 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/Template.php(455): __TwigTemplate_c5943fef62a01b10b3a47fe42b774c67a6a2d78a6b0ba5672f86b0ff6a0ee652->doDisplay(Array, Array) #5 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/Template.php(422): Twig\Template->displayWithErrorHandling(Array, Array) #6 /var/www/clients/client1/web16/web/grav/cache/twig/e4/e4e174049892ac3ae362b18566924bd7e8f186b67c9c3b91c7f5475d32340462.php(47): Twig\Template->display(Array, Array) #7 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/Template.php(455): __TwigTemplate_836cbd2454ca2a3d5c45d351cab74883d96c90315732160e3217c92a93229886->doDisplay(Array, Array) #8 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/Template.php(422): Twig\Template->displayWithErrorHandling(Array, Array) #9 /var/www/clients/client1/web16/web/grav/cache/twig/8e/8e203720bc9d53281b4de3287a4e5e09dc3b0b47ce3c3722f554df5985df1390.php(35): Twig\Template->display(Array, Array) #10 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/Template.php(455): __TwigTemplate_283f3bf2cacd9344fc38fe0cef8629bcbe41a1e97909b5c2f3c73cc0928088c1->doDisplay(Array, Array) #11 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/Template.php(422): Twig\Template->displayWithErrorHandling(Array, Array) #12 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/Template.php(434): Twig\Template->display(Array) #13 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/TemplateWrapper.php(47): Twig\Template->render(Array, Array) #14 /var/www/clients/client1/web16/web/grav/vendor/twig/twig/src/Environment.php(384): Twig\TemplateWrapper->render(Array) #15 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Twig/Twig.php(388): Twig\Environment->render('chapter.html.tw...', Array) #16 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Service/OutputServiceProvider.php(28): Grav\Common\Twig\Twig->processSite('html') #17 /var/www/clients/client1/web16/web/grav/vendor/pimple/pimple/src/Pimple/Container.php(118): Grav\Common\Service\OutputServiceProvider->Grav\Common\Service\{closure}(Object(Grav\Common\Grav)) #18 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/RenderProcessor.php(28): Pimple\Container->offsetGet('output') #19 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\RenderProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #20 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #21 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/DebuggerAssetsProcessor.php(28): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #22 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\DebuggerAssetsProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #23 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #24 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/PagesProcessor.php(69): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #25 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\PagesProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #26 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #27 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/TwigProcessor.php(27): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #28 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\TwigProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #29 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #30 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/AssetsProcessor.php(28): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #31 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\AssetsProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #32 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #33 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/SchedulerProcessor.php(29): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #34 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\SchedulerProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #35 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #36 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/BackupsProcessor.php(28): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #37 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\BackupsProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #38 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #39 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/TasksProcessor.php(59): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #40 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\TasksProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #41 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #42 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/RequestProcessor.php(52): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #43 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\RequestProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #44 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #45 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/ThemesProcessor.php(27): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #46 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\ThemesProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #47 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #48 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/PluginsProcessor.php(30): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #49 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\PluginsProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #50 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #51 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/InitializeProcessor.php(95): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #52 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\InitializeProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #53 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #54 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/DebuggerProcessor.php(27): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #55 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\DebuggerProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #56 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #57 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/ErrorsProcessor.php(27): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #58 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\ErrorsProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #59 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #60 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/LoggerProcessor.php(48): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #61 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\LoggerProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #62 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #63 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Processors/ConfigurationProcessor.php(28): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #64 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\ConfigurationProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #65 /var/www/clients/client1/web16/web/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #66 /var/www/clients/client1/web16/web/grav/system/src/Grav/Common/Grav.php(272): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #67 /var/www/clients/client1/web16/web/grav/index.php(54): Grav\Common\Grav->process() #68 {main} [] []

Where to begin troubleshooting?

What is the meaning of the multiple occurring string “handle(Object(Nyholm\Psr7\ServerRequest)”?
I do not have a Nyholm Psr7 server installed here.
Do I need it at all? Apache2 runs here.