I did remove and put back the black ink cartridge and printed an email. No squares.
Nice troubleshooting, congratulations! It seems that either the cartridge wasn't properly seated or the cartridge contacts were not fully contacting. For some reason Lexmark frequently advises users with 'blank page' printing issues etc. to (carefully) clean those contacts with a pencil eraser, although simply reseating the cartridge might fix a (minor) contact problem. Perhaps that also applied to your situation.
