There are several issues with the Majestic Q400, but most of them are related to the status of the engines, and this can be fixed by pressing CTRL+Shift+F1 (default key for fuel valves shutoff), so that GSX can detect engines off.
Maybe, your problem it's just that.
If you have moved the "Verbosity" slider in GSX too low, GSX is trying to tell you you can't use it because engines are turned off, but you can't see the message because you suppressed all the warning messages that way.