Hi Owen,
It appears that my problem was just a newbie mistake after all. Near the end of last week (Thursday or Friday, it must have been), I decided to try out Hyper-V on my machine, just to see what it could do. I didn't end up doing anything with it, but I didn't get rid of it, either. I never drew the connection until today, when I decided to try running my LilyDev installation on Hyper-V instead of VirtualBox as a last resort. It was pretty slow, but make passed without errors. Then I finally realized that two VM systems on the same machine might not actually play well with each other! *facepalm* I uninstalled Hyper-V (and WSL, which has been sitting unused on my computer for a few months) and uninstalled/reinstalled VirtualBox. Sure enough, everything works perfectly again. Thomas, Jean, Michael, and Jonas, I'm sorry to have wasted your time like this. I appreciate how willing you've been to help me out, though, and I hope you all have a great rest of your summer (or winter for those in the Southern Hemisphere).
No worries. I would not call this time wasted at all - I like this kind of detective work and it is a opportunity to learn new things. Thanks for working on LilyPond! Michael
