Backup to NAS or a different activity(replication, validation) fails with:
Windows error: (0x80070544) The validation information class requested was invalid
The share cannot be mounted from a particular Windows machine using the supplied credentials.
The username should be specified in hostname\username format, i.e. have target server name prefix. E.g if NAS name is BackupStorage and your username on that NAS is backupadmin, use Backupstorage\backupadmin as the username.
In case providing credentials in the specified format does not help, try accessing the same share using native Windows Explorer (from Windows machine which you try to backup) - this would help to troubleshoot the issue and find the proper username format.