At one of our sites someone added some Virtual Machines to one of the Veeam jobs. The new VMs failed backing up with the error message :-
Error: RPC Function call failed. Function name: [IsSnapshotInProgress]. Target machine [MACHINE IP]. RPC error: The remote procedure call failed and did not execute. Code: 1727
On checking the individual machines we found that the Windows Firewall was on for these machines. We disabled the firewall and they all started working the next run.
It seems to be due to the RPC ports being blocked.
Source : Veeam KB