repeated network-related panics/lock-ups on 8.1

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

It looks that a server of mine will consistently lock up or panic as soon as 
I run an Icinga 2 master on it (which will fork /a lot/).
I had the Icinga 2 master run on the same machine before, when it still ran 
6.1, without problems. I moved it back to this machine after upgrading to 8.1 
and it will lock up while I'm trying to enter the password for the console 
login.

I could enter ddb, backtraces were:
sbappendaddr
udp6_sendup
udp_realinput
udp_input
ip6intr
softint_dispatch
DDB lost frame
Xsoftintr

sbcompress
sbappendstream
tcp_input
ipintr
softint_dispatch

sbdrop
sofree
in_pcbdetach
tcp_close
tcp_input
ipintr
softint_dispatch

sbdrop
sbrelease
in_pcbdetach
tcp_close
tcp_input
ipintr
softint_dispatch


I also got a panic: sbdrop(0xfffffe840c125d68,20): cc=20
vpanic
snprintf
sbflush
tcp_input
ipintr
softint_dispatch

How can I debug this? The system is unsusable for me in this state.