Desktopcouch doesn't replicate with json_encode error 500

The bookmark-sync feature of UbuntuOne doesn't work for me on two different computers running Ubuntu Maverick. I already tried using a new account after cleaning all desktop-couch related files - suspecting some serverside error - but it still doesn't work.

However, the feature is - of course - enabled and replication works well for contacts and tomboy-notes. Only the bookmarks and a db called '_users' are affected.

Apparently the db's exist on server and client - but can not be replicated. The error given in desktop-couch-replication.log is (long lines, therefore separated by a blank line for readability):

2010-10-22 11:18:28,382 DEBUG asking 'http://localhost:35108/' to replicate {'url': 'https://couchdb.one.ubuntu.com/u%2F1f8%2F8d3%2F738677%2Fbookmarks', 'auth': {'oauth': {dbus.String(u'consumer_secret'): 'HiddenHiddenHiddenHiddenHidden', dbus.String(u'token'): dbus.String(u'cNJsnqBUWobwNhGndYCHmirQGJfMpVWUgTmyFZqzVyeQodKMJb'), dbus.String(u'consumer_key'): dbus.String(u'JHntcJE'), dbus.String(u'name'): dbus.String(u'Ubuntu One @ karpov'), dbus.String(u'token_secret'): 'HiddenHiddenHiddenHiddenHiddenHiddenHiddenHiddenHi'}}} to bookmarks

2010-10-22 11:18:29,302 ERROR can't replicate 'u/1f8/8d3/738677/bookmarks' 'http://localhost:35108/' <== {'source': {'url': 'https://couchdb.one.ubuntu.com/u%2F1f8%2F8d3%2F738677%2Fbookmarks', 'auth': {'oauth': {dbus.String(u'consumer_secret'): 'HiddenHiddenHiddenHiddenHidden', dbus.String(u'token'): dbus.String(u'cNJsnqBUWobwNhGndYCHmirQGJfMpVWUgTmyFZqzVyeQodKMJb'), dbus.String(u'consumer_key'): dbus.String(u'JHntcJE'), dbus.String(u'name'): dbus.String(u'Ubuntu One @ karpov'), dbus.String(u'token_secret'): 'HiddenHiddenHiddenHiddenHiddenHiddenHiddenHiddenHi'}}}, 'target': 'bookmarks'}
Traceback (most recent call last):

  File "/usr/lib/pymodules/python2.6/desktopcouch/pair/couchdb_pairing/couchdb_io.py", line 288, in replicate
    content=record)
  File "/usr/lib/pymodules/python2.6/couchdb/client.py", line 985, in post
    **params)
  File "/usr/lib/pymodules/python2.6/couchdb/client.py", line 1035, in _request
    raise ServerError((status_code, error))
ServerError: (500, ('json_encode', '{bad_term,<0.10712.0>}'))

For the _users-db the error is different (here the short version, see log for full message):
2010-10-22 11:18:30,185 INFO Connecting to http://localhost:35108/.
2010-10-22 11:18:30,186 ERROR can't create/verify '_users' None:35108 oauth=None

complete desktop-couch-replication.log is attached.