After installation of Windows Server 2012 R2 Update KB2919355, backups of Hyper-V VM’s fail. The error is
Error: Client error: The system cannot find the file specified
This error is not caused by the backup software nor the storage used. It is a bug in the Microsoft update. Veeam is curently working on a workaround which is expected to be released this Tuesday.
So be carefull installing this Microsoft update.
More info on the Veeam forum.