Connection between Desktop app and Tacx Neo 2T drops out

I have bought a new trainer Neo 2T, a brandnew PC where I installed the Desktop App. I can pair the trainer with the app but fairly quickly after doing a ride it loses the trainer and it is not displayed then anymore on the upper right side of the app. If I search for it again it would find and connect to it again.

a.) I have put my cell phone on flight mode --> no interference

b.) Mouse and keyboard of my PC are wire-connected

c.) ANT+ dongle T2028 is very close to trainer (< 1m)

Does anyone have an idea what could cause this problems? My PC runs with latest Windows and has Bluetooth 5.0 if I am not mistaken.

Garmin Express or any similar app is deinstalled to avoid interference.

Thanks for supporting me.