Need help: Backup Exec fails while using vss writer. Error 0x800423f4.

Dear Ladies and Gentlemen,

we have a several problem while using Backup Exec on Windows Server 2008 R2 Enterprise. A Backup from our Hyper-V Virtual Server (even Windows Server 2008 R2 Enterprise) fails. In the Logfile we read: VSS Writer fails with error (0x800423f4), timed out. After vssadmin list writers we are unable to identify the writer. Only one writer has an error:

Verfassername: "ASR Writer"
   Verfasserkennung: {be000cbe-11fe-4426-9c58-531aa6355fc4}
   Verfasserinstanzkennung: {8ff445e7-844a-487a-8ce6-f906a85738cc}
   Status: [7] Fehler
   Letzter Fehler: Zeitüberschreitung

How can we fix this problem?

Thanks for help.

Sincerely,

Christian Zöpfchen