Did you check the status of the writers before the backups?
vssadmin list writers
and ensure that all writers are present (including Hyper-V writer) and that all of them are in status  stable.
thanks for the hint. Microsoft Hyper-V VSS Writer is in state waiting for completion.
This happens sometimes after some successful backups.
I have no solution yet.
I would suggest you to take a look at the Application and System event logs, and as usual, please run NMM Config Checker 1.1 and find out if there is any Microsoft path/hot fix missing.
Please note that for NMM backups the best practice, and even a requirement is to set the group time out to "0", as well as the client retries, which should be also set to "0".
Anyway I would suggest you also to set the TCP/IP best practices in client and server.
If you find this behavior constantly (Hyper-V writer left in status  waiting for completion) than maybe you would like to open a case for further investigation, but please check the Application and System event logs in case you can find more information about this condition.
Good morning Bernd!
Microsft is famous for having VSS issues. I have worked with many backup applications and the issues tend to be the same. Because of these issues, I have complied a short list of the most common Microsoft VSS issues. They are:
Windows Server VSS issues:
Below are some articles that should be reviewed by a Windows server administrator. These articles pertain to VSS and possible issues that may occur.
Knowledge base articles - known Windows 2003 VSS issues:
A Volume Shadow Copy Service (VSS) update package is available for Windows Server 2003 - Last Review: July 2, 2010
Exchange VSS backup fails on an Exchange Server 2003 server - February 20, 2010
Various issues may occur on a Windows Server 2003-based computer that is running the Volume Shadow Copy Service - April 27, 2010 - April 27, 2010
Exchange Server 2003 data backup and Volume Shadow Copy services
Knowledge base articles - known Windows 2008 VSS issues:
VSS snapshot creation may fail after a LUN resynchronization on a computer that is running Windows 7 or Windows Server 2008 R2 - November 25, 2009
Backup fails with VSS Event ID 12292 and 11 on Windows Server 2008 and Windows Server 2008 R2 - January 20, 2010
No VSS writers are listed when you run the “vssadmin list writers” command in Windows Server 2008 R2
Windows 2008 R2 64-bit backup failed
General VSS articles to review:
The backup process may fail and a time-out error may occur in Volume Shadow Copy Service writers
Event ID 9840 or 9607 is logged when a VSS backup operation fails in Exchange 2007 or in Exchange 2003
Best Practices for Using Volume Shadow Copy Service with Exchange Server 2003
Error message: "Not enough server storage is available to process this command"
Please take the time to read through these articles and ensure they apply to your enviroment BEFORE implementing any of the changes.
Let us know if this helps.