I can't repro this on my own Big Sur machine

Saving seems to be fine for me on 11.1, what what it's worth.

Downloaded and installed on macOS Big Sur, and so far it's working great.

[~/.uqm]

May be interesting to know that while I have a ~/.uqm folder, there's no uqm.cfg file within it. Just 2 empty folders named save and teams. Not sure if that is expected for a new install.

Creating uqm.cfg manually with the line you shared does fix my problem though!

I recently updated macOS to the latest version (Big Sur). When I try to install the latest Ur Quan Masters, the game will launch fine, and I can hear the background music, etc., but the screen is a white blank, whether in full screen or windowed mode.

I tested in a macOS Mojave virtual machine and it works fine there.

Not sure whether the same issue occurs in Catalina, since I don't have a copy installed anywhere to test.