I must say these kind of mappings are a bit beyond me.
I'd try to see whether a managed entity of that type serializes & deserializes back alright. Can you also provide the exact ehcache version you're using?
So I can map that stack trace back and see whether that hints to anything further. My suspicion is that the CacheKey isn't properly serialized, i.e. serializes that SerializableProxy and that Stream can't be read back.
Steve Ebersole does that ring a bell with you by any chance?