QGC gets MAV Link Data but not Vehicle Connection

Hi, for a while now i tried to set up a mavlink router system.The Holybros s500 PX4 i work with spontaniualsy stopped working.All i did was changing some parameters in the MAV Tab and QGC only shows me the incomming stream via MAV Link Inspector but not a connected Vehicle. (Windows connects sound ,disconnect sound, connect sound)

So far i tried:
MP connection ,failed
Another USB Cable,failed
Another USB Port, failed
Diffrent PC , failed
Reinstalling QGC ,failed

An this happend twice. I stopped working with the first PX4 and used another one but after a day the same failure accured. I can buy new ones but i have the slight feeling that this happens again.