You say that bbb isa aaa, and that aaa isa bbb. That makes no sense, and it's the root cause of that error.

Update: Oops, you already knew that (and have since updated your node to say so explicitely). Ignore above.

Update: So I dug some more. Based on the message "Recalcing overload magic in package XXX", I checked overload. There's definitely a reference to () in the source, relating to the fallback special overload. It doesn't seem to be called by overload itself.