After recently bringing up a RODC in my default site, all my 2003 RWDC's in all my sites flipped to a single process which is not a good thing for DC's. I can't be absolutely certain this was the cause but the errors occured on the same day of the RODC promotion.
The erorr in DCDiag should look like something similar to below:
WIN32_OWN_PROCESS, expected value WIN32_SHARE_PROCESS
You can easily fix this either by running an SC command or from RegEdit. I would recommend SC just to ensure that it is properly configured.
From a command prompt key in the following. Make sure that you include the "Space" after the "=" otherwise you will get an error.
sc config rpcss type= share (Remotely -> sc \\RemoteServer config rpcss type= share)
If you want to do this manually then open up the Registry Editor and drill down
The value should be Equal to 10 (Hex) if the single process is configured and 20 (Hex) if Multi-Process is configured