Check to see if some SQL databases have been added to/or removed from the protected SharePoint farm. If this is the case, ...

1) Check to see if some SQL databases have been added to/or removed from the protected SharePoint farm. If this is the case, go to the protection pane and stop protecting this farm with the option to retain current data backups. Then, recreate protection for this SharePoint farm.
2) Check to see if any of the SQL data bases belonging to this farm is already protected as a SQL data source by DPM. If this is the case, then go to the protection pane and stop protecting this SQL database with the option to delete the replica. After this is done, recreate protection for this SharePoint farm.