The logs indicate that a user with the role "Student" had updated the grades from other users, but i don't believe it for the following reasons:

Users haven't the permissions:
The users have the role student and a detailed check showed, that they have no rights to change something.
Users don't see settings /gradebook etc.
I logged in as the concerned users and there were no possibilites or links to get in the grade book.

I suspect an issue in the Logs. We know that the grades didn't actually change. So what could have triggered the log to show this? We are using the "standard log". The event looks like this:

The user with id 'xxx4' updated the grade with id 'xxxxx4' for the user with id 'xxx4' for the grade item with id 'xxxx8'.

Also of note - these grade items were manually created.

Thank you for your help and consideration.
-David