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
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\RpcSs\Type
The value should be Equal to 10 (Hex) if the single process is configured and 20 (Hex) if Multi-Process is configured