Update: I could implement a workaround: The error only occurs, when the navigating away from an URL with PHP Get request in the address. That’s why it always worked when changing from month to list view and vice versa, as mentioned above. Now I changed the naviagtion link to the Events Calendar to http://www.eeb-sued-west.de/wordpress/events/anstehend/ and it works.
The case is closed for me, but I still find it odd a little.