Firstly, a big thank you for the suggestions and advice. I opened up the 861 over the weekend (paying special attention not to remove the screws that hold the card runners) and reseated the cards. I powered it back up before putting the case back together and was relieved to see that the Bad Code message disappeared from the front.

I closed everything back up and it is working fine again. When I had it open, I spent some extra time on the two buried cards CD/EF pulling them out and reseating them a few times. Visually, I did not see any oxidation or junk, but very happy to have everything working again.

Once again, thanks to all for your help and suggestions!