Only squares instead of letters and numbers are displayed in my calculator and other apps in Ubuntu 18.04

I don't know what I did wrong in my Ubuntu 18.04 OS. For some reason some of my apps look like this:

This also happens to other gnome apps like pdf-reviewer and Logs. Window title, menu bar etc. are all blank, display only squares.

*Problem solved: I downloaded Google Fonts, saved them in a new folder, they can be applied system-wide, but after I did some font settings using Google Fonts it didn't work. I changed the font of Interface with Tweaks-Tool, that was what causing the problem.

Solution 1:

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

Solution 2:

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)

Solution 3:

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.