FreeFileSync hangs and can not be cancelled

[hd@hdelfs.de]

Computer with Windows 10 pro 20H2 64bit, FFS version 11.6 64bit, Norton 360

I use FFS to daily synchronize my user data on a separate disk. After successfully synchronizing some 100000 files FFS hangs with 13 files remaining for unlimited time. Applying the Stop button the activity shows that some files are moved to the bin. Then nothing. The program cannot be cancelled. It must be finished via taskmanager.
After restarting the program shortly shows 13 files remaining but after one or two seconds is finished.
I think, there are probably a few locked files, but I cannot find out which files.
What can I do?

[Zenju]

I can have a look: https://freefilesync.org/faq.php#crash