APRIL 2013: This app is no longer being maintained.
Google have begun disabling sync access from any apps that are not explicitly created by Google.
If you are getting "error 500" messages, you could try and put the app back to using the Android Account for authentication and you might get some more syncs out of it. But if Google have *explicitly* disabled api access from any app that doesn't have "Google" in the name, then I don't think it will take long before all other methods used by CMarks are similarly detected and rejected.
Please don't ask me if there are other ways to keep syncing - if there were, you'd be using it now.
Thanks for all the support over the last two and a half years - it's been fun!
16th April 2013: This app is no longer being maintained.