Thanks for the detailed error description. The problem for Conboy is definitively the error returned by U1. Unfortunately U1 think it's clever to return HTML to a JSON request...

The big question of course it, what causes that error and I must admit that I don't really know. It looks like the error already occurs at the authentication stage, so I think we can assume that it's not a problem of malformed notes.

Did you try to sync Tomboy with that account? Does it work?