Unfortunately, I have no idea. There are a couple of other CUDA environment variables, but they look right and deal with the toolkit I just installed.
My best guess is some other piece of software was misbehaving. I have a number of other 3D-oriented programs on this computer, like Blender and Quixel Mixer. Maybe it had something to do with MSYS.
If it ever happens again I will try to isolate it. Until then, the best I can do is if you have an issue like this, check to make sure you don’t have a CUDA_VISIBLE_DEVICES set.