Szulak: Could you please remove everything from the %appdata%/GOG.com path, and restart the GOG Downloader? There was some changes on our back-end, but everything should work fine. 
  Removed everything from appdata directory. After entering login details client says "Online" and "Connecting". The following is in logger.log -  
 16:00:13 Login started 
 16:00:13 Requesting 
https://api.gog.com/downloader2/user/stable/ 
 16:00:13 Request data oauth_consumer_key=1e98d7a0b09c4a155b78cdb5ca09bee309355bb7&oauth_nonce=963074041&oauth_signature_meth od=HMAC- SHA1&oauth_timestamp=1375714704&oauth_token=1eeab34aa9b6d2ec257649d0fa369dc16b7d0fbf&oauth_version =1.0&oauth_s ignature=JY%2Bzha4S3pWFO4Ib3BKkv8NQ7hI%3D 
 16:00:13 Request 
https://api.gog.com/downloader2/user/stable/ successed?:True 
 Message: Cannot convert null to a value type. 
 Stack trace: at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeMain(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) 
 at System.Web.Script.Serialization.ObjectConverter.AssignToPropertyOrField(Object propertyValue, Object o, String memberName, JavaScriptSerializer serializer, Boolean throwOnError) 
 at System.Web.Script.Serialization.ObjectConverter.ConvertDictionaryToObject(IDictionary`2 dictionary, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) 
 at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeInternal(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) 
 at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeMain(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) 
 at System.Web.Script.Serialization.ObjectConverter.AssignToPropertyOrField(Object propertyValue, Object o, String memberName, JavaScriptSerializer serializer, Boolean throwOnError) 
 at System.Web.Script.Serialization.ObjectConverter.ConvertDictionaryToObject(IDictionary`2 dictionary, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) 
 at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeInternal(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) 
 at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeMain(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) 
 at System.Web.Script.Serialization.ObjectConverter.AssignToPropertyOrField(Object propertyValue, Object o, String memberName, JavaScriptSerializer serializer, Boolean throwOnError) 
 at System.Web.Script.Serialization.ObjectConverter.ConvertDictionaryToObject(IDictionary`2 dictionary, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) 
 at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeInternal(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) 
 at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeMain(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) 
 at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToType(Object o, Type type, JavaScriptSerializer serializer) 
 at System.Web.Script.Serialization.JavaScriptSerializer.Deserialize(JavaScriptSerializer serializer, String input, Type type, Int32 depthLimit) 
 at System.Web.Script.Serialization.JavaScriptSerializer.Deserialize[T](String input) 
 at GOGCompanionApp.AppWindow.LoginUser() 
 Inner exception: