Problem still exists even after disabling apparmor completely:
May 14 09:46:18 dc sh[4241]: ../../../../lib/isc/unix/socket.c:3361: INSIST(!sock->pending_send) failed, back trace
May 14 09:46:18 dc sh[4241]: #0 0x7fdeeb0dca4a in ??
May 14 09:46:18 dc sh[4241]: #1 0x7fdeeb0dc980 in ??
May 14 09:46:18 dc sh[4241]: #2 0x7fdeeb1187e1 in ??
May 14 09:46:18 dc sh[4241]: #3 0x7fdeeaebf609 in ??
May 14 09:46:18 dc sh[4241]: #4 0x7fdeeaffb103 in ??
May 14 09:46:18 dc systemd[1]: isc-dhcp-server.service: Main process exited, code=killed, status=6/ABRT
May 14 09:46:18 dc systemd[1]: isc-dhcp-server.service: Failed with result 'signal'.
Problem still exists even after disabling apparmor completely:
May 14 09:46:18 dc sh[4241]: ../../. ./../lib/ isc/unix/ socket. c:3361: INSIST( !sock-> pending_ send) failed, back trace server. service: Main process exited, code=killed, status=6/ABRT server. service: Failed with result 'signal'.
May 14 09:46:18 dc sh[4241]: #0 0x7fdeeb0dca4a in ??
May 14 09:46:18 dc sh[4241]: #1 0x7fdeeb0dc980 in ??
May 14 09:46:18 dc sh[4241]: #2 0x7fdeeb1187e1 in ??
May 14 09:46:18 dc sh[4241]: #3 0x7fdeeaebf609 in ??
May 14 09:46:18 dc sh[4241]: #4 0x7fdeeaffb103 in ??
May 14 09:46:18 dc systemd[1]: isc-dhcp-
May 14 09:46:18 dc systemd[1]: isc-dhcp-
isc-dhcp-server 4.4.1-2.1ubuntu5 amd64