Well I’m possibly way off beam, as I’m used to working in an AD environment where I can simply give a machine permissions, it’s going back a while remembering network mapping under the more common setup.
Going way back to starting with Win 3.x I still tend to trust command-line more than explorer and would at least be trying
NET USE Z: \\server\share /user:Name password /persistent:Yes
and checking the drive was accessible to Name… then seeing what happened running backups. Under AD Name would become Name@domain.tld in the command cited.
If it failed I might also try just using the UNC path as a target instead of mapping drives, it’d be something to do while waiting for someone who knows more than me to come along…