It's been a while, but we've recently come across a ridiculous error situation with DPM that caused a random subset of Protected Servers to just stop backing up.
Each Volume or resource would be in "Replica is inconsistent" state. You'd play the usual game of running consistency checks, or consistency & synchronisation but the job goes to "OK" for a while - but the ability to make a new recovery point is missing (eg the "Create a recovery point after synchronizing") option is unavailable, and a short while later a protected volume would return to "replica is inconsistent" state.
After much head scratching and monitoring, we realised it was very simple... the installation path for DPM includes a temp folder, for example:
"C:\Program Files\Microsoft System Center 2012\DPM\DPM\Temp"
The folder "MTA" had been removed as part of a clear up of old temporary resources, and despite this folder not being actively used during a backup it seems not having it breaks DPM.
Simply recreate a folder called "MTA" and you'll then find everything is working just fine again - re-run those consistency checks and then make a recovery point with synchronisation and all will be well.
Hopefully this will help someone else with a similar issue!
