It is happens to me too, when I try to sync a lot of files or huge files and something goes wrong (sync was interrupted) like:
- network outage,
- Laptop going into sleep,
Files will remain locked by old session for some time. Usually it gets self repaired after some time - like sync could be completed next day.
Happens a lot, when syncing with external shares and NC server was not powerful enough to handle in time all files/updates/sessions to external share and files getting locked. Also will be unlocked after some time.