Consider the following scenario:
Recovery point creation jobs for Microsoft Hyper-V \Backup Using Saved State\W2K8-R2 on protected_VM have been failing. The number of failed recovery point creation jobs = 1.
If the datasource protected is SharePoint, then click on the Error Details to view the list of databases for which recovery point creation failed. (ID 3114)
DPM encountered a retryable VSS error. (ID 30112 Details: VssError: The writer experienced a transient error. If the backup process is retried, the error may not reoccur. (0x800423F3))
This problem occurs because there is no available space on the disk that is used for Web caching. When there is no available space on the disk, System Center Data Protection Manager cannot create a shadow copy.
For all the details including the steps to resolve this issue see the following Data Protection Manager Knowledge Base article:
KB981948 - "DPM encountered a retryable VSS error" error message when you use System Center Data Protection Manager 2010 or System Center Data Protection Manager 2007 to protect a Hyper-V virtual machine that has Forefront TMG 2010 installed
J.C. Hornbeck | System Center Knowledge Engineer