Runtime Error when saving a page in Admin Plugin

I’ve installed GRAV on a virtual server on Ubuntu 20.04. Everything seems to be working well except an issue in the Admin Plugin which prevents me from saving any pages. I can’t find any help in existing topics on the web.

I’m using a custom theme and the twig file mentioned below actually exists in the modular folder: /home/_carousel -> modular/carousel

Also the exact same website works on my local machine like a charm.

THE ERROR MESSAGE:

Twig \ Error \ RuntimeError
An exception has been thrown during the rendering of a template (“Template “modular/carousel.html.twig” is not defined.”).

Does anybody have a clue how to solve this?