Note that the USB stick was still plugged in, so no idea whether it tried booting off the internal disk or USB/miniroot.
I powered the machine down by holding the power button and booted it again, still with USB plugged in, but no trap occured this time. So this time I could enter PMON again as earlier, follow "Lemote systems final steps" from https://ftp.openbsd.org/pub/OpenBSD/snapshots/loongson/INSTALL.loongson by doing PMON> set bsd /bsd PMON> set ShowBootMenu no PMON> set al /dev/fs/ext2@wd0/boot/boot and reboot through `reboot' in the PMON prompt; after that, still with USB plugged in, it booted up to the "login: " prompt just fine. So something seemed off upon this very first reboot, but I cannot reproduce it so far.
