You might want to check this post (and mabey subsequent posts referred there)
http://scn.sap.com/thread/3167996
Bottom line is: Check "Trace" section in SXMB_MONI in receiver system. There you should be able to get more details about the error causing the exception to be risen.
You might try a search with those error details from "Trace" section. If you are not able to resolve, come back.
Could also be possible that you need do debug the proxy class, but since this is standard code, I don't think that this is actually a bug in the code but some missing / wrong customizing. Debugging however, could lead you to the place where the exception is risen so you might be able to determine the cause. I would resort to debugging however only if error details in "Trace" are not sufficient to resolve the problem.
HTH
Cheers
Jens