it seems like the
/etc/fonts/fonts.conf file went missing. Maybe you changed the environment variable
FONTCONFIG_PATH, which should be pointing to that file, or simply the file was deleted. Please check the file exists and is accesible by Discourse.
If it didn't work, please check this thread from ask ubuntu forums.
If, finally, nothing worked at all, please come back and I'll try to find another workaround.