Got this error flooding our SharePoint environment. Seems to happen when the user profile sync does not sync properly with the content DBs. Run the command below to resolved. The command DOES NOT delete any content DBs, just the invalid sync data:
stsadm -o sync -deleteolddatabases n
Where n is number of days since last sync (trial and error here)
More info here: http://support.microsoft.com/kb/2771190 – if the KB still exists when you read this 🙂