If you're running RelayNodeClient.jar it looks like it's time to upgrade. I got the following email from the author, Matt Corallo:
Sorry for the inconvenience, please upgrade your relay client node as an
incompatible server change was made

. Still, fitting most blocks in
one TCP packet is probably worth it.
I'm waiting to hear back from him to see if he will be producing a .jar file, right now there's only RelayNodeClient.java.
If you're in conversation with him, can you ask whether the jar file is even needed? Seems like putting it in the bitcoin.conf does the trick as well, or are there things within the jar that specifically aren't available by just peering using "addnode"? If it's fine just peering, like PatMan said, one less thing to have to worry about especially with the associated java bloat.