I was recently quoted $100 by Canon to fix the common AF problem on that lens. I opened mine up first too, what a mess, very fiddly stuff, and there was a broken gear so wasn't worth the hassle for me to fix it at that price. Had it back in a bit over a week, good as new.
I suppose if any part of the lens electronics is not talking to the camera, you'll get an error. That's gonna be focus position, aperture, MF switch?.. Not sure what else. A little static damage could kill components in there quite easily too.
ONE reason I like 30 year old Nikkor lenses.