If Moonlight complains about an app currently running, force closing the app should fix the issue.
If Sunshine isn't working properly, you can try restarting it. This will terminate any running sessions.
If Sunshine is stuck trying to restore the changed display device settings, you can reset the settings and proceed to restore the display state manually. This could happen for various reasons: device is no longer available, has been plugged to a different port and so on.
Remove your paired devices.
See the logs uploaded by Sunshine