Thanks again SeveriniAnthony and NeoStrider!
I tried a render with the default light on, no change. The card was still there.
I just tried a test render with the card removed at exactly the spot it was supposed to disappear, and it works fine. I will rerender the last 400+ frames without the *$^#(% card! (Since the camera was locked, the tiffs match up perfectly).
One odd thing...after deleting the card, I find I cannot save the scene (even after rebooting). I will have to check, but I may have constrained the card to a referenced object (the doctor character) rather than on the original. Maybe that has something to do with it! In any case, it won't matter, as the next scene is a completely different shot and the following scene will need to be recomposed anyway.
Whatever the case, this is a perfectly good workaround which doesn't require Herculean effort, so I will keep this in my bag o' tricks should it ever happen again!
Thanks again, shipmates!!!!
"Ad astra per aspera..."