The solution for me using fujifilm xt3 & the fuji x webcam app for zoom was setting shutter speed to 30, ISO to auto, aperture 0 on the camera. Also make sure video settings on camera are set to 25 or 30 fps.
Before I found this solution - I had the same issues with the fuji webcam app greying out and not launching, on zoom the fujifilm placeholder card was showing instead of the camera feed. But when I changed the shutter dial to 30, the camera feed just started working I didn't even need to launch the fuji webcam app.
I think its because zoom etc is looking for the frame rate input to its system to match what the camera is telling it, so if you set the shutter speed to auto (as I have seen recommended on tutorials for this) its not recognising the input