Thanks, Catlady, no idea why, but that did it. What had me scratching my head was that the file was readable to Firefox/EPUBReader. I wasn't sure if this was an idiosyncrasy of the JBL or what.
I don't know that it matters, but I generally put the SD card in a card reader and move files directly to it from the computer, then I close it up and put the loaded card in the JBL. If this happens again I'll try to take a closer look at the file to see if anything has been corrupted.
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