This is tricky to troubleshoot as you end up waiting days for the error to crop up again on the DPM Server
run this command to force the catalog creation
Get-ProtectionGroup yourdpmservername |get-datasource |where-object {$_.type -like "*sharepoint*"} | start-createcatalog
Sources :
http://www.itwalkthru.com/2010/10/dpm-2010-and-sharepoint-2010-metadata.html
http://www.itwalkthru.com/2010/06/dpm-powershell-force-sharepoint-catalog.html