“Watched” Status suddenly on all movies

Hi everybody,
I encountered a strange behaviour in KODI 16 some weeks ago. Suddenly all movies were marked as “watched” without any input from my side.
I am running KODI for a long time (since XBMC days) on a Windows 7 64bit machine without problems. No Soft- or Hardware changes at that time.

How can this happen? (There’s no cat in the house…). Where is this information stored, is it in one of the xml or txt files? Can this be restored without a valid backup?
Trying to unmark the “watched” status is not possible, the check mark stays when I try to remove it, so that is really strange.

Any suggestions?

Thank you all in advance for tips and tricks…

Burz