Hi Natalia,
Yes, so we have a course that has a forum with Turnitin disabled and when we back up and restore the course, the forum is working for user but when we want to edit the settings, the error pops up even though the forum has Turnitin disabled. I also tested a few things:
Adding a new forum to the restored course, same error pops up when creating.
Did not include the forum when back up, restore to a new course then create a new forum, same error pops up.
Creating a new forum in another old course, no error…
It seems a bit random to me…Also, the course does not even have a Turnitin activity or assignment activity.