Deleting “InstantOn.driver” at /Library/Audio/Plug-Ins/HAL/“ and a reboot solved my problem.

And the problem is not limited to AirPods. Since I upgraded to Big Sur 11.0.1 none of my audio devices work (Onkyo earphones and Bose Soundlink Mini II). They connect and disconnect immediately. I have tried several remedies proposed here or elsewhere, nothing seems to work. Reverting back the OS solves the problem, so it is purely Big Sur problem.