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.