Backups to the Local Authentication Manager Server or NFS are working.
The changes made as part of supporting the NTLMv2 in SP1 Patch 1 enforced Authentication Manager to use the security options ntlmssp or ntlmsspi. If your Windows Shared Folder does not support these security modes, then Authentication Manager will fail to mount the share and fail to perform the backup.
Apply Authentication Manager 8.1 SP1 P8 or later to solve the problem as per defect AM-29331.
As workarounds, try the following:
Use NFS backup,
Use internal backup, or
Change the security mode to NTLMv2 on the Windows machine.