I just updated my system and it is not fixed yet, or released yet. But I found some more information that might give some more clues on what actually happens. Because it is not just people that are disconnected from the internet, it affects also people that did not sign up for the ubuntu one service as well, whether they are connected to the internet or not.
If I turn off the ubuntu one store, Banshee works fine, but as soon as I hit the ubuntu store option it starts spinning at 341% (!) CPU (on a quadcore, most probably it just starts using all available processor power), and Banshee is completely stuck. (I once tried to leave it for a while, but that just drained my battery)
This is the debug output that immediately arises when I hit the option. It searches for a folder that does not exist on my computer. Despite the warnings that occur, it apparently continues nevertheless, trying to resolve a NULL somewhere....

(Banshee:3546): libsoup-WARNING **: No feature manager for feature of type 'U1RequestChrome'
[1 Debug 16:02:26.942] Extension source loaded: Ubuntu One-muziekwinkel

** (Banshee:3546): WARNING **: Error calling get_info: Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/dbus/service.py", line 702, in _message_cb
    retval = candidate_method(self, *args, **keywords)
  File "/usr/lib/python2.7/dist-packages/ubuntuone-client/ubuntuone/platform/linux/dbus_interface.py", line 1041, in get_info
    return self.syncdaemon_folders.get_info(path)
  File "/usr/lib/python2.7/dist-packages/ubuntuone-client/ubuntuone/syncdaemon/interaction_interfaces.py", line 640, in get_info
    mdobj = self.fs.get_by_path(path.encode('utf-8'))
  File "/usr/lib/python2.7/dist-packages/ubuntuone-client/ubuntuone/syncdaemon/filesystem_manager.py", line 781, in get_by_path
    mdid = self._idx_path[path]
KeyError: '/home/bdekoning/.ubuntuone/Purchased from Ubuntu One'


** (Banshee:3546): CRITICAL **: syncdaemon_folder_info_get_subscribed: assertion `SYNCDAEMON_IS_FOLDER_INFO (finfo)' failed
** (Banshee:3546): DEBUG: Loading the real store page

** (Banshee:3546): WARNING **: Got less number of items in credentials hash table than expected!

** (Banshee:3546): WARNING **: Error rescanning Purchased Music: Bestand of map bestaat niet

(Banshee:3546): GLib-CRITICAL **: g_str_has_prefix: assertion `str != NULL' failed
[1 Debug 16:02:37.459] U1MS: Url Loaded: 
Marshaling url-loaded signal
Exception in Gtk# callback delegate
  Note: Applications can use GLib.ExceptionManager.UnhandledException to handle the exception.
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object
  at Banshee.UbuntuOneMusicStore.UbuntuOneMusicStoreSource.OnDefaultStoreUrlLoaded (System.Object o, UbuntuOne.UrlLoadedArgs args) [0x00000] in <filename unknown>:0 
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0 
  --- End of inner exception stack trace ---
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0 
  at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in <filename unknown>:0 
  at System.Delegate.DynamicInvokeImpl (System.Object[] args) [0x00000] in <filename unknown>:0 
  at System.MulticastDelegate.DynamicInvokeImpl (System.Object[] args) [0x00000] in <filename unknown>:0 
  at System.Delegate.DynamicInvoke (System.Object[] args) [0x00000] in <filename unknown>:0 
  at GLib.Signal.ClosureInvokedCB (System.Object o, GLib.ClosureInvokedArgs args) [0x00000] in <filename unknown>:0 
  at GLib.SignalClosure.Invoke (GLib.ClosureInvokedArgs args) [0x00000] in <filename unknown>:0 
  at GLib.SignalClosure.MarshalCallback (IntPtr raw_closure, IntPtr return_val, UInt32 n_param_vals, IntPtr param_values, IntPtr invocation_hint, IntPtr marshal_data) [0x00000] in <filename unknown>:0 
   at GLib.ExceptionManager.RaiseUnhandledException(System.Exception e, Boolean is_terminal)
   at GLib.SignalClosure.MarshalCallback(IntPtr raw_closure, IntPtr return_val, UInt32 n_param_vals, IntPtr param_values, IntPtr invocation_hint, IntPtr marshal_data)
   at Gtk.Dialog.gtk_dialog_run(IntPtr )
   at Gtk.Dialog.Run()
   at Banshee.Gui.Dialogs.BansheeDialog.Run()
   at Banshee.Gui.GlobalActions.OnPreferences(System.Object o, System.EventArgs args)
   at System.Reflection.MonoMethod.InternalInvoke(System.Reflection.MonoMethod , System.Object , System.Object[] , System.Exception ByRef )
   at System.Reflection.MonoMethod.Invoke(System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)
   at System.Reflection.MethodBase.Invoke(System.Object obj, System.Object[] parameters)
   at System.Delegate.DynamicInvokeImpl(System.Object[] args)
   at System.MulticastDelegate.DynamicInvokeImpl(System.Object[] args)
   at System.Delegate.DynamicInvoke(System.Object[] args)
   at GLib.Signal.ClosureInvokedCB(System.Object o, GLib.ClosureInvokedArgs args)
   at GLib.SignalClosure.Invoke(GLib.ClosureInvokedArgs args)
   at GLib.SignalClosure.MarshalCallback(IntPtr raw_closure, IntPtr return_val, UInt32 n_param_vals, IntPtr param_values, IntPtr invocation_hint, IntPtr marshal_data)
   at Gtk.Application.gtk_main()
   at Gtk.Application.Run()
   at Banshee.Gui.GtkBaseClient.Run()
   at Banshee.Gui.GtkBaseClient.Startup()
   at Hyena.Gui.CleanRoomStartup.Startup(Hyena.Gui.StartupInvocationHandler startup)
   at Banshee.Gui.GtkBaseClient.Startup()
   at Banshee.Gui.GtkBaseClient.Startup(System.String[] args)
   at Nereid.Client.Main(System.String[] args)
   at System.AppDomain.ExecuteAssembly(System.AppDomain , System.Reflection.Assembly , System.String[] )
   at System.AppDomain.ExecuteAssemblyInternal(System.Reflection.Assembly a, System.String[] args)
   at System.AppDomain.ExecuteAssembly(System.String assemblyFile, System.Security.Policy.Evidence assemblySecurity, System.String[] args)
   at System.AppDomain.ExecuteAssembly(System.String assemblyFile)
   at Booter.Booter.BootClient(System.String clientName)
   at Booter.Booter.Main()