Failure trying to synch site for ContentDB WebApp

Problem:
Periodical errors in the event log of a SharePoint 2010 server with the following error event descriptions:
Failure trying to synch site for ContentDB WebApp . Exception message was Cannot insert duplicate key row in object 'dbo.UserMemberships' with unique index 'CX_UserMemberships_RecordId_MemberGroupId_SID'. The statement has been terminated.
Cause:
Old references to non existing content databases in the synchronization job list.

Fix:
List databases which are not synced for more than two days
stsadm -o sync -listolddatabases 2

Delete old database references from the config db
stsadm -o sync -deleteolddatabases 2