Complete IO lockup, state "ufs" from userland, debuging help wanted

[Harry Schmalzbauer]

 Hello,

I can easily lock up FreeBSD stable/11 from userland. Not that I want to...
I'm running squid, which starts an authentication helper
"*negotiate_kerberos_auth*", which seems to be the culprit.
Completely all IO is blocked, there's no way to get anything from any
filesystem.
All non IO-requesting processes(threads) run well, including sshd and
shells.
There's no load (neither cpu nor io) just any process requesting io
stucks in state "ufs"

Can anyone help me finding out what's going wrong?
Serial console is available.

Thanks in advance,

-harry