One final thing you can try.
Get a pan and put a soft paper towel in the bottom.
Fill with 1.5" of very hot (not boiling) water.
Remove all the ink cartridges, then remove the printhead assembly.
Gently put the printhead nozzles down in the water and let it soak for five minutes.
Remove the printhead from the water and gently dry the electrical contacts.
Do not touch the nozzle area.
Put the printhead back in the printer and then reinstall the ink cartridges.
If the error is gone then run up to three cleaning cycles.