Hello Davy,
I downloaded the latest version, built it and tried it as follows:
      // Manually logging the exception
      Exception exception = new ArgumentNullException();
      ILog log = LogManager.Adapter.GetLogger(this.GetType());
      log.Info(exception.Message, exception);
      // Let Agatha log the exception
      throw exception;
When I manually log the exception I get it right in the database:
  
  Message: Value cannot be null.
  StackTrace: System.ArgumentNullException: Value ...
  Type: System.ArgumentNullException
When the exception is logged by Agatha in fact I get 2 logs in the database with the following messages:
1 - RequestProcessor: unhandled exception while handling request!
     This one has the same message as usual ... StackTrace and Type are ok.
2 - System.ArgumentNullException: Value cannot be null.     at 
MVCApp.ServiceLayer.Handlers.CountUsersByDateHandler.Handle(CountUsersByDateRequest
 request) in ...
     This one has the entire Stack Trace in the message. StackTrace and Type are empty.
I think you might missed something when doing the change ... no?
Thank You,
Miguel