While using the Classic UI, I upgraded to 24.1.1.0, and it completely broke the menus. The entire ribbon section was blank, and none of the Alt keys would open a menu. See attached screenshot.
I shut down and restarted the new version multiple times, which did not help. I had to manually change ShowOldToolbars in FileVoyager.ini to 0, then relaunch FV. After the relaunch, I could enable/disable the Classic UI via preferences w/o getting any errors.