Hello Henning,
Greetings from North Idaho, US.
It looks like 2 lines were changed in the slotforms.php file (a call to require_once for the form module and a tweaked/simplified "if clause" later on). This has indeed eliminated the symptoms we were experiencing and the Scheduler activity appears as desired even when not in edit mode. The Apache error log is also clean (no PHP warnings or errors).
Thank you very much for the fix. Our teachers will be delighted.
Sincerely,
rob