I have to bring this topic up again.
As it seems impossible to get kresd working again, I am stuck with Unbound.
The issue is still there. It seems that occasionally, but at least every time a device enters the network (gets powered on or awoken from sleep) the issue occurs again.
unbound-control spawns zombie processes when /etc/resolver/dhcp_host_domain_ng.py is called which brings my network to a halt for about a minute or two.
Any more ideas on how to fix this? This time unbound-control itself is causing the issue.