APRIL 2013: This app is no longer being maintained.
Google have started 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.
If you have the Full app, use CMarks to manually backup your bookmarks to an html file or a pdf on Google Drive and then use either the html or your Drive account as your browser's homepage - at least then you should still see your current set of bookmarks. If your browser doesn't have that ability, I can't help there!
If you purchased the app any-time after GMT Midnight 1st April 2013 then I am willing to offer a refund - email me your original order number from your purchase or the email address you used to purchase from. I can't offer refunds for any longer - I've already allowed more than the mandatory 15 minutes refund window by two weeks out of the kindness of my own pocket and Google would agree that 15 minutes is more than enough for all refunds.
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.