Actually there was not much else to see in the page, only the error ;)
But full developer-mode helped to identify the block that was causing trouble. (I must have taken the one below before)
Strangely the block ist used in all courses! I deactivated the plugin (Notifications Block) and now the course works.
I wonder how to deactivate the block only for that course, because I cannot remove the block when it is there, because the page won't load, and neither when the plugin is completely deactivated, because then I cannot remove the single instance of the block...
But thanks already, this helped a lot!