Backup data not work after update to 6.8

I’ve updated a server to 6.8 but since that day the data backup doesn’t work.
ERROR - Error while mounting 192.168.1.180:BackDir : mount error(5): Input/output error Refer to the mount.cifs(8) manual page (e.g. man mount.cifs)

The “BackDir” is hosted on a Qnap Ts-253Pro with the last firmware, set on SMB 2,1 compatibility.

How to test manually why the mount fail ?

Edit: also with nethserver 6.9 the backup doesn’t work, the mount of the SMB share, fail.