So, as of yesterday night, without any noticeable change to the setup, my Pi2/retropie has failed to boot. Consistently.
Even with reformatting the card and rewriting the disk image. I've considered that this could be sign of a corrupted sd card, so I got a new one, but the issue persists! Swapped usb pen drives too - and I think it's more related to that in particular -, but it still happens.
It just displays the following text:
what(): boost::filesystem::create_directory: No space left on device: "/home/pi/.emulationstation/gamelists/amstradcpc"