@GoSharks: Yup.
From what I gathered from the logs, the initial culprit seems to have been a snafu in cover mode: cache hit at boot on the last book opened (possibly inaccurate because of date/time issue, hence always the same across reboots), and that cached cover was broken (which, in a perfect world, should never happen with stuff generated by the hack, I can only think of an issue because of a weird/broken source image, or a bug in IM).
The log didn't go that far back, so the fact that I saw mostly cover mode logs might be a complete coincidence
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I'm not sure switching to and fro cover mode would have helped anyway, nothing is ever deleted, so the broken image always has ways to get back in the pool.