Comment # 15 on bug 1197422 from Hannes Reinecke

But in either case: I've bought brand-new SD cards with the same make than the
manufacturer-provided one (SanDisk Ultra 32GB), wrote the most current JeOS
image to it, and, voila, system booted.

So either the old card was dodgy to begin with (always a possibility, given
that it's one I found in one of my drawers), or the Unmatched firmware is very
picky about which SD card to accept (and the documentation hints at something
like that).

Anyway. System boots now.