It was Kaspersky Internet Security which had a question in the background.. Couldn't saw it because I couldn't switch to desktop. I only saw that the mouse cursor was changing to a pointing hand at the left downer side of the screen - I clicked there and switched to desktop where I could see that KIS blocked the use of the microphone for ETS2. Because of ETS2 was set to trusty in KIS this can't be the problem. So I looked how TruckersMP.exe is set and saw that it was weakly limited. So I went into KIS Settings -> application management and there I set the use of microphone to allow in the KIS settings for truckersmp.exe
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8i11y's post in 6 Minutes till Loginwindow appears was marked as the answer