The downloader is failing for me, I found these in the logs, PTAL. Its not allowing me to add with https:// removed // in the log sample below. 
 
 22:25:00 Login started 
 22:25:00 Requesting https: api.gog.com/downloader2/user/stable/ 
 22:25:00 Request data oauth_consumer_key=<...>&oauth_nonce=<...>&oauth_signature_method=HMAC-SHA1&oauth_time stamp=137580 8055&oauth_token=<...>&oauth_version=1.0&oauth_signature=<...>%3D 
 22:25:01 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.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: 
 Closing Downloader 
 Serializing application state. 
 Setting LastRunError to false
Post edited August 06, 2013 by harishjp