I mentioned GDT corruption as a possible cause, and it was pointed out
to me that OpenBSD had fixed a bug in my GDT code (derived from the
i386 version) that fixed corruption. And this was indeed the case,
so I applied the change.

You need include/gdt.h revision 1.2 and amd64/gdt.c revision 1.5.

Please let me know if it fixes the issue. i could easily reproduce
the problem by starting more than 512 processes (causing the GDT
to grow beyong its original boundary).

- Frank