I am having the same error. I know my credentials and can use them to log in on the website. Ubuntu One was working until today. Now it tries to connect, connects successfully, and then immediately disconnects.

I have tried deleting the login key, deleting my computer from the service and reinstalling ubuntu one according to the directions on canonical's site.

The error shows up in syncdaemon-exceptions.log :

2014-02-20 19:39:07,221 - ubuntuone.SyncDaemon.Main - INFO - hash queue empty. We are ready!
2014-02-20 19:39:07,249 - twisted - ERROR - Unhandled error in Deferred:
2014-02-20 19:39:07,250 - twisted - ERROR - Unhandled Error
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 381, in callback
    self._startRunCallbacks(result)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 489, in _startRunCallbacks
    self._runCallbacks()
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 576, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1127, in gotResult
    _inlineCallbacks(r, g, deferred)
--- <exception caught here> ---
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1071, in _inlineCallbacks
    result = g.send(result)
  File "/usr/lib/python2.7/dist-packages/ubuntuone-client/ubuntuone/syncdaemon/interaction_interfaces.py", line 1273, in connect
    raise NoAccessToken("got empty credentials.")
ubuntuone.syncdaemon.interaction_interfaces.NoAccessToken: got empty credentials.

The relevant lines of the syncdaemon.log are:

2014-02-20 19:48:05,097 - ubuntuone.SyncDaemon.ActionQueue - INFO - The request 'caps_raising_if_not_accepted' finished OK.
2014-02-20 19:48:05,254 - ubuntuone.SyncDaemon.ActionQueue - INFO - The request 'oauth_authenticate' failed with the error: TRY_AGAIN and was handled with the event: SYS_SERVER_ERROR
2014-02-20 19:48:05,256 - ubuntuone.SyncDaemon.StorageClient - INFO - Connection lost, reason: [Failure instance: Traceback (failure with no frames): <class 'twisted.internet.error.ConnectionLost'>: Connection to the other side was lost in a non-clean fashion: Connection lost.
].

and of credentials.log :

2014-02-20 19:49:39,687 - ubuntuone.credentials - DEBUG - login: args (<ubuntuone.platform.credentials.CredentialsManagementTool object at 0xaea5ccc>,), kwargs {'window_id': 0}.
2014-02-20 19:49:39,937 - ubuntuone.credentials - INFO - Starting Ubuntu One login manager for bus 'com.ubuntuone.Credentials'.
2014-02-20 19:49:39,942 - ubuntuone.credentials - DEBUG - ref_count is 0, changing value to 1.
2014-02-20 19:49:40,149 - ubuntuone.credentials - DEBUG - Handling DBus signal for member: 'CredentialsFound', app_name: dbus.String(u'Ubuntu One').
2014-02-20 19:49:40,149 - ubuntuone.credentials - INFO - 'CredentialsFound'
2014-02-20 19:49:40,149 - ubuntuone.credentials - DEBUG - ref_count is 1, changing value to 0.
2014-02-20 19:49:40,149 - ubuntuone.credentials - DEBUG - Setting up timer with <function timeout_add at 0xb6ca4e64> (10000, <bound method CredentialsManagement.shutdown of <ubuntuone.platform.credentials.dbus_service.CredentialsManagement at /credentials at 0x87c56ec>>).
2014-02-20 19:49:40,149 - ubuntuone.credentials - INFO - CredentialsManagement: emitting CredentialsFound.
2
2014-02-20 19:49:40,150 - ubuntuone.credentials - DEBUG - cleanup: removing signal match <<class 'dbus.connection.SignalMatch'> at ae939ec "type='signal',sender='com.ubuntuone.Credentials',path='/credentials',interface='com.ubuntuone.CredentialsManagement',member='CredentialsFound'" on conn <dbus._dbus.SessionBus (session) at 0xab0a6ec>>
2014-02-20 19:49:40,150 - ubuntuone.credentials - DEBUG - cleanup: removing signal match <<class 'dbus.connection.SignalMatch'> at ae938cc "type='signal',sender='com.ubuntuone.Credentials',path='/credentials',interface='com.ubuntuone.CredentialsManagement',member='AuthorizationDenied'" on conn <dbus._dbus.SessionBus (session) at 0xab0a6ec>>
2014-02-20 19:49:40,151 - ubuntuone.credentials - DEBUG - cleanup: removing signal match <<class 'dbus.connection.SignalMatch'> at ae93b6c "type='signal',sender='com.ubuntuone.Credentials',path='/credentials',interface='com.ubuntuone.CredentialsManagement',member='CredentialsError'" on conn <dbus._dbus.SessionBus (session) at 0xab0a6ec>>

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: ubuntuone-client 4.2.0-0ubuntu1
Uname: Linux gk 3.8.0-35-generic
Architecture: i686
UpgradeStatus: Upgraded from 12.04
LANGUAGE: en
LANG: en_US.UTF-8
SHELL: /bin/bash