The best thing to do is enable logging and see if you can get a better error message.
To enable logging:
1. Navigate to C:\Program Files\Microsoft Dynamics ERP\Management Reporter\2.1\Server\Services\
2. Open the MRServiceHost.exe.config in notepad.
3. Search for <add name="Microsoft.Dynamics.Integration.TraceLog" value="0"/>
4. Change the 0 to a 4, so it looks like this: <add name="Microsoft.Dynamics.Integration.TraceLog" value="4"/>
5. Restart the MR Process Service.
The log file will capture the error and it may provide some more input to what is failing.
Most of the time it is inconsistent data.