Issues with known valid characters displaying as rectangles sometimes and not others

[Ofnuts]

Either:

1) you lost a font (but in that case there are fall-back ones... at least if the UI is in English). Hardly klikely
2) Gimp is trying to use a different UI language

Re-installing won't fix anything since the problem is likely in your Gimp profile and this is kept across installations. The first thing to so is to remove your profile (by renaming it so you can copy old contents) so that Gimp creates a new one.