Here is a similar error, the backup destination couldn’t be reached:
Could it be that the NS8 backup of samba can’t access the NAS because the NAS can’t access samba on NS8 to check users/permissions?
Maybe test the backup on a guest writable share on the NAS?
Another idea:
Do all the timeouts occur after the same time?
Maybe we can use some timeout option like --contimeout=TIME, see also Documentation