Did you check the depth of your cameras? (it defines the order in which they are rendered).
Also, I would type “Camera” in the search bar of the hierarchy window to locate any camera you could have forgotten or added by mistake.
I think I’m having the same issue. I have 3 cameras in my scene and the main one that’s rendering my game renders black until I change a setting on it in the editor. It was working just fine in 2018 and I’m not sure what’s going on when fixing it is literally just “change a setting in the editor and it starts working again”.
Found a solution for my case !
In Camera variables, there’s a dropdown for Target Eye for VR, and it was set to Both.
Changing it to None (Main display) did the trick for me !