Ok, the laptop is finally up and running again. But what a journey!
I'm still not hundred percent sure what happened, but my guess is that #kdediscover (with integrated support for #lvfs) initiated a #bios update without me noticing the day before. And as I was in a hurry and distracted, I accidentally hard-reset the machine during the update that ran on the next boot and therefore corrupting the flash content.
I guess I will disable LVFS as update source in Discover for now...