I have now successfully installed an ASUS PCE-AX1800. Here the bluetooth works...

@woozle thank you so much for your really great tips and help and effort! No, I don't have an external hardrive or anything connected via cable. Yes, unfortunately, I don't see anything else I could...

Last thing I tried: I disconnected my (working) usb-port and connected the cable to this usb. Bluetooth shows on, but still connecting devices is impossible. I begin to suspect there is something...

@woozle thank you so much for your answer! Yes, I had another bluetooth device (a tplink usb stick) installed before. I did what you recommended and removed the hidden bluetooth entries in the device...

I installed the "TP-Link Archer TX20E AX1800 Wi-Fi 6 Bluetooth 5.2 PCIe-Adapter" according to installation guide (and connected the usb on the motherboard correctly). The wifi works fine. Bluetooth...