Recently I have resolved an issue which is almost same configuration and the same error. Then, I updated TECH61192.
"An unknown error occurred" may occur when backing up to tape on a server with the HP Server Management Agents Software installed.
...
We have found a performance issue in the Checkpoint Restart.
Please disable the Checkpoint Restart in the Backup Options > Advanced Open File.
http://www.symantec.com/docs/TECH191072
Thanks.
The issue is caused by some change in Windows Server 2008 R2.
Currently, Symantec finds out that the message can be ignored. We are still investigating if the issue can be resolved by an application level.
Thanks