Only squares instead of letters and numbers are displayed in my calculator and other apps in Ubuntu 18.04
Correct font permission issues, then rebuild the font cache:
find /usr/share/fonts -type f -exec sudo chmod 644 {} \;
find ~/.fonts -type f -exec sudo chmod 644 {} \;
sudo fc-cache -r -v
It seems that you configured a Font which does not include the necessary glyphs to display. With a look to your name... do you use a Chinese font?
Maybe changing the font will help you. (see Font Settings Design)
The 3 line previous answer doesn't work on 20.04, or at least didn't on mine. It looks like they moved the folder.
I used dconf reset -f /
and just reset the desktop to default.