sounds like there may be an issue with some oxidation or other dirt on the contact pins that touch that big array on the back-- get some contact cleaner (deoxit etc.) or even isopropyl alcohol if you have any on hand, and with a non-woven cloth, or ideally a little foam wiper stick that they sell for print head cleaning, wipe the pins in the head socket in the downwards direction ONLY. I've had quite a few head errors resolve this way. Also i've found on a number of occasions i'll get a head error when the blue ink in particular has clumped up on the head, which i have been able to clean successfully with printhead cleaner and the error will go away. Sounds like through all those changes you finally were able to make good contact with whichever pin(s) is/are dirty.
