I'm trying to set up Tomboy notes sync with Ubuntu One. I've followed the instructions in this guide. I get this successful authorization screen:

And this is what my sync preferences screen looks like:

When I click "Save" in the sync preferences I get this success screen:

But when I click "Yes" I get this failure screen:

I think this is the relevant error from my .xsession-errors file:

[ERROR 09:38:25.560] Caught exception. Message: The remote server returned an error: (404) NOT FOUND.

[ERROR 09:38:25.561] Stack trace for previous exception:   at System.Net.HttpWebRequest.CheckFinalStatus (System.Net.WebAsyncResult result) [0x00000] in <filename unknown>:0 

   at System.Net.HttpWebRequest.SetResponseData (System.Net.WebConnectionData data) [0x00000] in <filename unknown>:0 

[ERROR 09:38:25.561] Rest of stack trace for above exception:    at System.Environment.get_StackTrace()

   at Tomboy.WebSync.Api.OAuth.MakeWebRequest(RequestMethod method, System.String url, System.Collections.Generic.List`1 parameters, System.String postData)

   at Tomboy.WebSync.Api.OAuth.WebRequest(RequestMethod method, System.String url, System.String postData)

   at Tomboy.WebSync.Api.OAuth.Get(System.String uri, IDictionary`2 queryParameters)

   at Tomboy.WebSync.Api.UserInfo.GetUser(System.String userUri, IWebConnection connection)

   at Tomboy.WebSync.WebSyncServer.BeginSyncTransaction()

   at Tomboy.Sync.SyncManager.SynchronizationThread()

   at System.Threading.Thread.StartUnsafe()

[ERROR 09:38:25.561] Synchronization failed with the following exception: The remote server returned an error: (404) NOT FOUND.

  at System.Net.HttpWebRequest.CheckFinalStatus (System.Net.WebAsyncResult result) [0x00000] in <filename unknown>:0 

  at System.Net.HttpWebRequest.SetResponseData (System.Net.WebConnectionData data) [0x00000] in <filename unknown>:0

And the full output of tomboy --debug is here.