OH, okay, that is unexpected, but makes more sense.
The UFO files on my affected computer do have icons that show a Fontra logo, rather than the usual UFO icon, so that is probably another side effect.
I now see the related thread Cannot read UFO files if macOS UTI is not “Unified Font Object” - #8 by GeorgSeifert .
I’ve tried installing an older RF beta, and even just the stable non-beta RF, and doing a computer restart, but I’m still unable to open UFOs in Glyphs.
Maybe it is also partially due to Fontra? My UFO icons have returned to normal, but possibly the UTI is still mixed up.