My case might have been different, but during my “out-of-syncs” saga #13488 (my problem #2), I had a case where a computer (Workstation) reported failed items and if I pressed the folder-specific rescan, the rescan took unrealistically short time for such a large folder and did not appear to be doing anything. I had to press “rescan all” and only then a real rescan started for that particular folder and the “failed items” were cleared away.