Cannot delete file sync.ffs_lock. Error code 5: Access is denied. (DeleteFile) error

[gana2k1]

Hi
I have noticed on very rare occassions receiving this specific error when doing one way sync to my qnap drive. But after upgrading to the latest 7.1, it is happening almost every other day.
I was hoping is there a way around it? And I dont know if it is a FFS issue or something in QNAP not releasing the file?
My details:
1. FFS on Windows
2. QNAP drive mapped
3. Nightly scheduled backup from Windows to QNAP - one way sync only
4. When this error happens, if i try to manually delete the sync.ffs_lock from the qnap drive from within Windows explorer, it looks as if the file is deleted, but if I refresh or dir again, it comes back - so it is not removing the file.
5. The only remedy is to restart QNAP completely (I am new to QNAP so there maybe another way that I am unaware of).

Any help would be appreciated.

Thanks

[gana2k1]

Anyone?
For now I have disabled lock file creation but would like to know how to resolve it properly.