From the descriptions it sounds like a kernel issue, but without some form of log evidence we really have nothing to research. If you remove "quiet" from kernel boot params in syslinux.cfg it will dump the system log onto the screen during boot. Record the screen as a slow-motion movie and as long as your hands are steady it's normally possible to play the resulting movie back and read the text to spot error messages. Upload load the movie to a sharing site somewhere and we can see it too.
NB: If LE 12.0 works on the boxes (but not LE 12.2) stick on the earlier release until kodi-headless has a K22 release available. The diff between Kodi versions isn't large so it's not a 'must have' update.