After installing Big Sur (at last, Safari 4K HDR!), the dreaded spinning beachball of doom appeared anytime I tried doing anything at all in Final Cut Pro 10.5. Unplugging my UltraStudio 4K Mini, things returned to normal, so I was certain the device was the culprit. I’d already updated to the latest 12.0 beta firmware, but this pop-up appeared every time I launched Desktop Video:
However, when I clicked on ‘update’, I got the following message. 😳
I tried uninstalling the Desktop Video drivers and installing the app again, after which A/V Output in Final Cut was greyed out. If this should happen to you, here’s the solution:
Do not install Desktop Video 12.0 Beta 1 Update unless your machine is running the new Apple M1 processor. Install ver. 11.7 instead.
The Daejeon Chronicles 
This was a life saver! Thanks!
Glad I could help.