Sorry for the delay in answering.
I have 3 CIFS repositories.
One of the repositories started failing authenticating.
Inside the same job about half of the VMs fail backing up due to login error.
I don't believe the problem is due to the NAS, because all the jobs should fail.
It's an intermittent error, so I considered restarting the jobs.
After restarting the jobs the problem was not solved and I am still investigating it.
I attempted creating a new job with no success.
Any idea to troubleshoot the problem?