I got so excited, thinking you hit the nail on the head! Alas, I'm still sorting it out, but I've at least narrowed it to Cura. I successfully started a print of an older model, previously printed, and that worked fine, which rules out the printer and the USB stick being bad, although I did reformat the USB stick. I tried processing the same original stl file with Cura, and it didn't work.
LAN and wifi aren't an option for me, unfortunately.
I noticed that the .gz extension was new, so on a whim I uninstalled Cura and installed an older version. It appears I'm back up and running. Not sure why, but now I have to get caught back up!
Thanks a bunch for your help!