Comment 3 for bug 875999

Fwiw I see exactly the same behavior in my oneiric box, the last few lines from the log (with --debug) are:
...
[DEBUG 14:16:55.220] Asking server for access token based on authorization token.
[DEBUG 14:16:55.220] Building web request for URL: https://one.ubuntu.com/oauth/access/
[DEBUG 14:16:55.327] Received response from server: oauth_token_secret=...
[DEBUG 14:16:55.327] Got access token from server
[DEBUG 14:16:55.327] Successfully authorized web sync
[DEBUG 14:17:05.161] GetConfiguredSyncService is null
[ERROR 14:17:05.164] Error cleaning up addin after sync: Object reference not set to an instance of an object
  at Tomboy.Sync.SyncManager.SynchronizationThread () [0x00000] in <filename unknown>:0
...