Same issue for me, Err70 whilst bracketing images on the 1Dx.
I might wait until I see if your repairs fix the issue before I contact Canon as I don't want a main board replacement for no reason.
Can you keep up posted if your issue is fixed or not?
I just got off the phone with Canon Repair in Irvine. They got notice on Friday that there is, indeed, a firmware issue causing this. Something to do with the timer and long exposures. ( ) The next firmware update (no ETA) should fix it. No hardware needs to be touched to fix it.
They are shipping my camera back today. Fortunately they didn't open her up since the parts weren't on hand anyway...
Since it's an easy workaround I'm good with that.