NTBackup (on WS2k3) fails to backup remote server (WS2k8R2) with " Error: is not a valid drive, or you do not have access."

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http://serverfault.com – We run an NTBackup job on a Windows Server 2003 R2 SP2 with all updates (as of Q4-2011). It works well backing up two WS2k3 servers as well as the backup server itself. However, we have been unable to successfully back up our Windows Server 2008 R2 machine. It often runs for about 2GB worth of backup and then dies out with one of the below error messages. It should be more like 20GB for the full server. We have tried using the admin share (C$), an explicitly shared drive share, UNC and mapped drives. The result is the same each time, the only thing that varies is the amount of stuff back (HowTos)