Check to see if some dependent data sources have been added to the protected data source %DatasourceName;. In that case run ...

1) Check to see if some dependent data sources have been added to the protected data source %DatasourceName;. In that case run "Modify Protection" for the protection group and consistency check for the data source by clicking the respective links below. 
2) If no dependent data sources have been added to %DatasourceName;, verify if any of the dependent data sources are already getting directly protected by DPM. If this is the case, then go to the protection pane and stop protecting these data sources with the option to delete the replica. After this is done, recreate protection for %DatasourceName;.
3) If neither of the earlier options are valid, verify if the associated VSS writers are running correctly on the backend Servers. If the writer is not in a healthy state, restart the writer.