KGPE-D16 bare metal testing

I ran a slightly modified version of live-bootstrap on an Asus KGPE-D16 with 2xAMD Opteron 6380, 128 GB of RAM and blob-free coreboot-15h, and booting from USB flash drive, and viewing output on an Acer monitor (VGA). It started bootstrapping with the usual text flying by for 5-8 minutes, and then I got blue flashing + - characters. Is this flashing screen part of the OS, the Bios, the monitor or what? My CPU fans still kept running and I kept it on like that for about 20 minutes before powering off (had to press the power switch because keyboard was unresponsive).

For reference I ran my own OS called PLX, which I'm trying to bootstrap with live-bootstrap. It worked fine on a VM.
PLX: https://github.com/bitmarkcc/plx/tree/amd64
LB: https://github.com/bitmarkcc/live-bootstrap

The report names live-bootstrap, PLX, coreboot, and the KGPE-D16 but no specific file, test, or failing command. Start by reproducing the bare-metal bootstrap while recording the last output before the flashing screen. Done means identifying whether the failure is in live-bootstrap, PLX, coreboot, or the display path and documenting a reproducible diagnosis.

Written by the indexing model from the issue text.