I have what appears to be a corrupt system font (.Geeza Pro PUA). How do I go about replacing it with a working version? iMac running 10.12.5.
It's doing this:
Open Font Book.app and choose File → Restore Standard Fonts….
You could probably swap a working version on the other macs to replace this corrupt one but the font name does suggest that the font uses Private Usage Area for specific characters, my knowledge even unicode doesn't have a standard of what should be encoded to that set. Don't use this font, buy or licence a font that works well. For a relatively well supported font I'd suggest Noto from Google