Several Problems/error messages after installation of Grav CMS + Adminpanel

#1

I have an 1&1 shared hosting account. I installed the new Grav CMS & Admin Panel software manually on this server account.

After this I tried to see the website, but there is an error:

E_ERROR - Class ‘Thunder\Shortcode\Shortcode\AbstractShortcode’ not found

After I logged into the admin panel I tried to open the links to: Configuration (ERROR: 400 - Template “partials/modal-changes-detected.html.twig” is not defined in “config.html.twig” at line 89.); Pages ( 400 - Template “partials/modal-changes-detected.html.twig” is not defined in “pages.html.twig” at line 395.). At Plugins I can see the list of installed plugins, but if I want to click on it the following error comes: 400 - Template “partials/modal-changes-detected.html.twig” is not defined in “partials/plugins-details.html.twig” at line 58.

What is wrong with the CMS software? It is terrible to install! In the past I had made pages with Joomla and Typo3. I wanna test this CMS and I would like to use it for a small page.

Please help!