Status 71 is due to none of the file exist and this can happen is different circumstances
1> If the file doesn't exist and you are trying to backup that file. Eg. If you are backing up c:\temp\nbu.txt in the backup and if the file nbu.txt is removed from that folder you will get status 71
2> If you are running a job with multi streaming on
And if for that server q doesn't exist it will give you that error message.
3>As you are backing up CIFS through AD server, what credentials NBU client using on AD server ?
Make sure NBU client service on AD server should be started with the credentials that has access to the cifs share. If it doesn't have access that will give you status 71