Disappearing directory/mount and Crash on umount since 5.3

[Vince Hoffman]

I'm having a reoccuring problem with one of my mounts,
I have a disk i mount at /usr/scratch which is also exported via samba.
since upgrading to 5.3 after a little while (sorry no exact timings 
yet,) an "ls /usr/scratch" gives "ls: /usr/scratch: Bad file descriptor"

root at lobster
(11:19:06 </usr>) 0 # mount
/dev/ad0s1a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0s1e on /tmp (ufs, local, soft-updates)
/dev/ad0s1f on /usr (ufs, local, soft-updates)
/dev/ad0s1d on /var (ufs, local, soft-updates)
/dev/ad2s1c on /usr/scratch (ufs, local, soft-updates)
/dev/ad3s1a on /big (ufs, local, soft-updates)
devfs on /var/named/dev (devfs, local)
root at lobster
(11:19:10 </usr>) 0 # ls
ls: scratch: Bad file descriptor
.snap/     bin/       home/      libdata/   obj/       share/
@LongLink  compat/    include/   libexec/   ports/     src/
X11R6/     games/     lib/       local/     sbin/      sup/
root at lobster

trying to umount /usr/scratch will crash the machine, (will do it once 
more this evening and write down the error that pops up.)

any ideas ?

Vince