MySQL error 1593 on replication between 5.0.92 and 5.5.42
You didn't setup the server-id variable, or you didn't restart after doing it (it is not a dynamic variable, changes will not be effective until you restart). It may be showing you a good id on both servers, but it will not really work until both servers are restarted, both master and slave.

It could be a bug, but the above has happened too many times on support cases that I would bet it is that. 

Regarding circular replication: 5.0 -> 5.5 "should work", 5.5 -> 5.0 is possible, but not recommended. I understand why you want that, but you may encounter many incompatibilities in the queries and format.