1. Check writers state with: vssadmin list writers in an elevated prompt (reset of some writers without reboot http://www.planetcobalt.net/sdb/vss_writers.shtml)
2. Check Application event log VSS entries
3. Check System event log for volsnap entries
4. Check for invalid service paths (http://blogs.technet.com/b/askcore/archive/2010/06/18/ps-script-for-blog-enumeration-of-the-files-failed.aspx)
5. If it is a cluster, check if there is a service installed on a shared drive, see the nice to know link
Often the problem is that the Shadow Area is too small, unfortunately this does not always show up in the event logs
Nice to know when troubleshooting VSS: http://blogs.technet.com/b/askcore/archive/2010/06/18/reasons-why-the-error-enumeration-of-the-files-failed-may-occur-during-system-state-backup.aspx