USBMS ate your babies.
c.f.,
https://github.com/koreader/koreader...mportant-notes
(I'll probably update the instructions to recommend a device reboot beforehand, as that should at least help prevent the Nickel side of things from screwing with this. That still leaves the truly terrible USB stack of various OSes to content with...).
TL;DR: Make sure the config is actually updated (re-run the OCP after a device reboot if need be); watch Nickel burn down all the things on unplug, then reinstall everything once again because Nickel just broke it
.