If you see this error on the primary DPM server then, 1) Check to see if you can reduce the number of express fulls (or file ...

If you see this error on the primary DPM server then, 
1) Check to see if you can reduce the number of express fulls (or file recovery points) for the data sources that are currently being protected
2) Reduce the retention range for the existing protection groups
3) Reduce the number of data sources being protected by this DPM server

If you see this error on the secondary DPM server then, 
1) Check to see if you can reduce the synchronization frequency for applications (or file recovery points) that are currently being protected
2) Reduce the retention range for the existing protection groups
3) Reduce the number of data sources being protected by this DPM server