Thanks for the reply, and aploogies for the delay.
I am attaching as requested a picture of the error message in the 2-line LCD display - it doesn't come out very clear, as the contrast is low, but it says "50.7 Fuser Error. Turn off then on". My manual has errors 50.1, 50.2, 50.3, 50.7, 50.8, 50.9 listed, and the description for 50.7 is "Fuser Open Error" - see copy of manual entry.
I also attach a picture of the DC controller board, and I have double-checked that all of the connectors shown are fully seated. I don't see any unconnected wires. Now, when I removed the connectors to get at the optics, some were difficult to remove and I had to be very careful to avoid damaging the wires into them, but as far as I can tell they all seem OK (but hard to tell for sure).