Tomboy cannot obtain lock from server and won't sync

Binary package hint: tomboy

I was trying to sync my notes via tomboy, without success.
I tried it via terminal too, and got the following error message:

[INFO 16:39:27.601] Initializing Mono.Addins
[ERROR 16:40:42.896] Synchronization failed with the following exception: Invalid syntax: Unterminated string, expected '"' termination, got ':' at [1:69772]
  at Hyena.Json.Tokenizer.InvalidSyntax (System.String message) [0x00000] in <filename unknown>:0
  at Hyena.Json.Tokenizer.LexString () [0x00000] in <filename unknown>:0
  at Hyena.Json.Tokenizer.InnerScan () [0x00000] in <filename unknown>:0
  at Hyena.Json.Tokenizer.Scan () [0x00000] in <filename unknown>:0
  at Hyena.Json.Deserializer.CheckScan (TokenType expected, Boolean eofok) [0x00000] in <filename unknown>:0
  at Hyena.Json.Deserializer.CheckScan (TokenType expected) [0x00000] in <filename unknown>:0
  at Hyena.Json.Deserializer.ParseObject () [0x00000] in <filename unknown>:0
  at Hyena.Json.Deserializer.Parse (Hyena.Json.Token token) [0x00000] in <filename unknown>:0
  at Hyena.Json.Deserializer.ParseArray () [0x00000] in <filename unknown>:0
  at Hyena.Json.Deserializer.ParseObject () [0x00000] in <filename unknown>:0
  at Hyena.Json.Deserializer.Parse (Hyena.Json.Token token) [0x00000] in <filename unknown>:0
  at Hyena.Json.Deserializer.Deserialize () [0x00000] in <filename unknown>:0
  at Tomboy.WebSync.Api.UserInfo.ParseJsonNotes (System.String jsonString, System.Nullable`1& latestSyncRevision) [0x00000] in <filename unknown>:0
  at Tomboy.WebSync.Api.UserInfo.GetNotes (Boolean includeContent, Int32 sinceRevision, System.Nullable`1& latestSyncRevision) [0x00000] in <filename unknown>:0
  at Tomboy.WebSync.WebSyncServer.GetNoteUpdatesSince (Int32 revision) [0x00000] in <filename unknown>:0
  at Tomboy.Sync.SyncManager.SynchronizationThread () [0x00000] in <filename unknown>:0

(Tomboy:7458): GLib-CRITICAL **: g_source_remove: assertion `tag > 0' failed
[ERROR 16:41:22.719] Caught exception. Message: The remote server returned an error: (500) INTERNAL SERVER ERROR.
[ERROR 16:41:22.719] 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 16:41:22.720] 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()
[ERROR 16:41:22.720] Synchronization failed with the following exception: The remote server returned an error: (500) INTERNAL SERVER ERROR.
  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

(Tomboy:7458): GLib-CRITICAL **: g_source_remove: assertion `tag > 0' failed

1,
I'm using ubuntu 11.04, natty. It was installed as gnome. Xubuntu was added from software center, and I am currently using it from xfce environment.

2,
apt-cache policy tomboy
tomboy:
  Telepítve: 1.6.0-0ubuntu3
  Jelölt: 1.6.0-0ubuntu3
  Verziótáblázat:
 *** 1.6.0-0ubuntu3 0
        500 http://ubuntu.mirrors.crysys.hu/ natty/main i386 Packages
        100 /var/lib/dpkg/status

3,
a sync via ubuntu one

4,
error msg, it has even crashed and disappeared once, but i couldn't reproduce that

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: tomboy 1.6.0-0ubuntu3
ProcVersionSignature: Ubuntu 2.6.38-8.42-generic 2.6.38.2
Uname: Linux 2.6.38-8-generic i686
NonfreeKernelModules: nvidia
Architecture: i386
Date: Fri Apr 29 16:44:06 2011
ProcEnviron:
 LANGUAGE=hu_HU:en
 LANG=hu_HU.UTF-8
 SHELL=/bin/bash
SourcePackage: tomboy
UpgradeStatus: No upgrade log present (probably fresh install)