r319971 -> r320351: Fatal error 'Cannot allocate red zone for initial thread'

Over the past week we did not update several 12-CURRENT running development
hosts, so today is the first day of performing this task.

First I hit the very same problem David Wolfskill reported earlier, a fatal
trap 12, but fowllowing the thread, I did as advised: removing /usr/obj
completely (we use filemon/WITH_META_MODE=YES all over the place) and
recompiling world and kernel.

Since tag 20170617 in /usr/src/UPDATING referred to the INO64 update and the
INO64 update hasn't performed so far starting from r319971, I installed the
kernel, rebooted the box in single user mode (this time smoothly), did a
mergemaster and tried to do "make installworld" - but the box instantanously
bails out:

[...]
Fatal error 'Cannot allocate red zone for initial thread' at line 392 in
file /usr/src/lib/libthread/thr_init.c
pid 60 (cc) uid0: exited on signal 6 ...

[...]

That way, I obviously can not install a world :-(

What is wrong here? Is the problem resovable?

Kind regards,

Oliver