I have been using video files with videoconferences for quite some time now and recently I also put it into a push-to-talk script. Only with Jitsi Meet it never worked - no matter which browser I tried - none could access my “webcam”. Today, after another naive search, I came across this github issue with the solution for Chrome.

You have to set the flag exclusive_caps to 1 when loading the module.

sudo modprobe v4l2loopback exclusive_caps=1

Now the “webcam” works in Chrome \o/ Still not in Firefox - at least in Chrome