Default HOX switching to airplane mode


my GF's phone keeps switching to airplane mode when she answers calls. She can talk for 5-25 second and then the phone switches to airplane mode. I allready checked if the sensor that turns the screen of isnt working, but that is not the problem. If i hide the sensor with my finger the screen turns off.
And she cant listen to voice messages through the speaker, in the moment she takes the phone to her ear, the messages stop.
Looks for me like an error with the sensor but as i said, when i hide it with my finger the screen turns off.
Its on Android 4.2.2 and not rooted or has any custom rom.
Maybee anybody had the same issue or an idea what could be the reason.
I thought about installing Viper one but i am not sure if this could fix a sensor error or if it would be the best solution to send it to htc.

Device: HTC ONE M7
ROM: ViperOne 4.3.0

My lovely Mr.singing club, what´s this for a juice store !