I have a system that has a very slow RAID that is also generally not very fast, and sometimes the filesystem would take quite some time to respond. Syncthing is a little bit too eager to declare the filesystem read-only (when it fails to chmod a file in what Syncthing thinks is a reasonable amount of time) and show the folder as out of sync. Even worse, Syncthing never retries these cases, not even upon rescans, so restarting the whole Syncthing seems to be the only way to resume normal operation.
A bug? An unfortunate combination of conditions? Some misconfiguration?