[CaptainTivo]

Error Writing the WL-database file [...] Incorrect datetime value

This might be a very brute-force solution ...:

Well... I got this error (incorrect datetime value) everytime and for each files. So, I deleted the database and started a new one. This seems to be good now.

Can you enable debug logging and look into the log file of Kodi? The popup notifications do not contain that much information.
If it is the same error as with the discussion on the strict mode (follow the link):

[...]Can you try to fix this in your Kodi database?[...]

OK, I deleted the database and recreated it and then ran Watchedlist.  It updated the database.  Then I changed a couple of TV episodes to "watched" and exited Kodi.  I then restarted Kodi and 5 minutes later, Watchedlist updated without errors.  So I guess that simply deleting the database and re-creating it fixed the problem.  Would still like to know what the problem was.  Thanks.