Just for information and to find the solution here if needed, I just had the same problem again.
Unfortunately, the paths at both ends of the synchronization were already identical.
Solved it as follows:
- stopped Syncthing at both ends
- manually changed a character in the paths at both ends
- Restarted Syncthing at both ends, no more error messages received
- stopped Syncthing again at both ends
- Changed paths at both ends manually back again
- Restarted Syncthing again at both ends, received no more error messages