The backup error VSS_WS_FAILED_AT_FREEZE may occur with several types of writers, such as the Hyper-V VSS Writer or Registry Writer.
We have a tool to diagnose all kinds of VSS errors, called VssDiag available at http://backupchain.com/VssDiag.html
The important thing is to enter the backup error time as accurately as possible, within a 5 minute window and the tool will find all relevant error details stored in Windows logs.
Most often, this error is caused indirectly by a different application or service, and chances are that service will also log its error in the Event Viewer. Because VSS is a framework where services, such as Exchange, SQL, and Windows itself, hook into, failures such as VSS_WS_FAILED_AT_FREEZE may be caused by a configuration or resource issue solely related to external service applications.
The freeze has to do with how long application “writers” have to hold their write access to disk. Usually that’s 60 seconds or less. Because some writers use a shorter timeout than 60 secs, the system may give freeze errors more often with disk I/O is high. Our VssDiag tool should be helpful in determining which service causes the problem. Most of the time it’s Microsoft Exchange, SQL Server, or the system registry service.
After examining the errors found by VssDiag, restart the services: COM+ System Application Service, Distributed Transaction Coordinator Service, and Volume Shadow Copy Service
You may also want to check our VSS Troubleshooting Guide at http://backupchain.com/hyper-v-backup/Troubleshooting.html