With the latest changes and improvements, the plugin now is clever enough to distinguish between the Opencast events related to the current StudIP instance or not.
This will lead to reject some events (if any) and not recording them in the db table when the discovery cronjob processes the events.
However, on the other hand the ACL sync cronjob still reads all the events from the Opencast and still assumes that all of them are recorded in db table locally, which leads to an error: Try to add created on null

The solution is only to check if the video record is there or not!

With that said, the PR fix follows.