TrueD VRx switches channel only sometimes and to random channels

[UnicornMoe]

I know that the TrueD Furious FPV isn't supported as a VRx yet. I tried to use my ESP8285 betaFPV lite receiver as a VRx Backpack anyway. As Firmware I flashed RX5808_ESP_RX_Backpack.
My Transmitter is the Happymodel TX Pro 2400 and has a built in Backpack on version 1.2.0
My Goggles are Fatshark Attitude V3.
I followed this wiring diagram https://github.com/ExpressLRS/Backpack/raw/master/img/rf-wiring0diagram.jpg connecting CLK to the boot-pad.

1. The VRx only switches channel sometimes and to random channels when I command it with the TX script. The command always arrives at the RX (it blinks 3x when I send a command). When it switches channel it sometimes immediatley switches one up afterwards.

Other things I found out:
2) With all 5 cables connected and the VRx installed in the goggles, the RX goes into bootloader mode (solid LED, not responsive to inputs from my radio). The VRx beeps once a second and switches one channel up and down (see this video: https://youtu.be/4T0QT9gV-JI ) This is caused by something inside my goggles, because it only happens if the VRx is installed in the goggles. Yes, I checked that there is no short between ground and the boot pad.

3. With the CLK cable disconnected and the VRx installed in the goggles, I can control the RX, for example when I swith channel with the ELRS script, the RX blinks and sometimes the VRx switches channel to a random channel. So the same behaviour as in 1)

Any Ideas what I could do?