Sure I can. Please see the screenshot with the configuration below. I think, however, that the issue i ran into was not so much about the specific integration of Pi-hole in TrueNAS, and would not hit a lot of other TrueNAS users.
I think it has to do with the fact how my interfaces an my server are configured. I bundle two physical interfaces to one logical interface (bond0) using LACP. I then had to apply the IP adress to the logical interface using an alias. Just an idea. I cannot quickly reconfigure switch & NAS to confirm.
