Thanks, Oscar,
Turned out to be quite simple for a change. Looked at my owner's manual, which I would not have done (it's usually pretty useless) had you not suggested it. There I found a line to the effect that the driver's side sliding door will not close if the fuel door is open (makes sense for safety reasons).
Well, my fuel door wasn't open, but when I looked it was slightly out of line. Opened it, did a little bending and, voila!, the sliding door now opens just fine.