@embryo: Ping @WS64 and/or open a GH issue.
Just one Q (the background info on your setup will be good to have no matter what): Do you actually use Calibre to send books to your device?
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I've seen *one* (apparently) successful OTA update in my server logs, so it at least appears to work correctly on Kindle devices, and zsync did indeed do its magic (311KB downloaded instead of 12.8MB

).