The error message was "Load paper in tray ___", when there already was paper in that tray, being properly stacked, and printer hardware working correctly (as later proven). This error was resolved by putting the computer onto a network and removing a custom size paper entry. The solution was provided by professionals at a large and high quality toner cartridge distribution company. However, I am sure I have never entered a custom size in this printer, and that I had previously inspected all printer settings for "custom" and found nothing. I did buy a printer that started life briefly on an organization's network, but could that network create the a "Custom" spec entry, when my printer on a USB line, did not show any custom entries remaining?
Is a custom paper size entered only in printing Preferences, Paper/Quality Tab, open the "Custom" box, and enter a size? Given that I have never done this or seen it in any printing shortcut, where was the custom size entry that caused the error message about needing to load the paper in a given tray? Is it possible that a "custom" size sheet had been entered in a network but no longer was visible with a USB line connection?