Hi,
thanks for your feedback!
Ok, did not look closely enough on the advanced tab - log level ist easy to overlook :)
Fortunately I was able to resolve my issue, though I still don't know what happened exactly.
I tried to take as much out of the equation as possible - my last try was to take even pfsense out...
So I started unbound as a docker container on one of my machines (pfsense still in use for internet gateway, though)...
That showed the exact same symptoms.
That was a serious WTF moment :|
I then googled for a long time, and tried to debug that single docker unbound instance - without success. It seemed as if something dropped packets, but I did not know what.
I eventually found a promising thread:
https://forums.freebsd.org/threads/unbound-very-slow-and-or-dns-address-could-not-be-found.57493/
That setup seemed to be similar. I as well use a Fritzbox that is tricked into working only as a dsl modem.
I tried to change the dsl protocol version as suggested in the thread - without success.
I then took a deeper look at the fritzbox again. There are various modes the fritzbox can be turned into a dsl modem.
I used the variant, where the fritzbox itself handles VLAN tag 7 for VDSL.
I found various threads, that this bridge mode suffers a drop-some-PPPoE-packets problem.
I then switched the box to full_bridge - of course I had to adjust pfsense to do the VLAN tagging itself on the WAN interface.
And guess what: it worked.
Unbound now works fine. Why the bridge mode of the fritzbox dropped just these specific DNS packets - I have absolutely no idea. Of course, I mustn't complain, since I use the box in a way it was not designed for (at least not officially) :)
So basically everything was set up correctly and I did look at the wrong end...