I use windows 7 and have redownloaded the lobby.
There seems to be a problem directly at start. Here the diagnostic_log:
08.09.2013 18:26:43: Unable to deserialize map Maptest34 from the server: System.Net.WebException: The remote server returned an error: (404) Not Found.
at System.Net.WebClient.DownloadDataInternal(Uri address, WebRequest& request)
at System.Net.WebClient.DownloadData(Uri address)
at System.Net.WebClient.DownloadData(String address)
at PlasmaShared.MetaDataCache.GetMap(String mapName, MapCallback callback, Action`1 errorCallback, String springVersion)
and unfortunately it still dosen't work. The same error message as previously occurs.