@RadimKozub I’ve had a similar issue, where my Librem 13 would crash intermittently. As mentioned by @mladen, disconnecting the battery and reconnecting after a while always worked for me.
The problem with my model pointed out by warranty support, was a rubber pad on the laptop back missing (must’ve misplaced it, really silly) which affected heat dissipation resulting in overheating.
Also, try upgrading to the latest version of coreboot (Librem coreboot Utility Script: usage, discussion, and help) if you haven’t already.
I haven’t faced this problem since and I don’t poweroff for weeks at times.
Merry Christmas!