$ gpg-connect-agent --dirmngr 'getinfo version' /bye
D 2.2.17
OK

thanks for the dns explanation - IMHO, there should be added something about that in the wiki
When it does not work for you on http1 either, then I guess, it's really just some outdatedness of my gpg/dirmngr and this ticket can be closed.

ok, I disabled it again. btw: why do we need openpgpkey.archlinux32.org in the cert? Is this standard or did I misconfigure something?

I set archlinux32.org back to http2 - so you can see for yourself, how gpg fails to retrieve the key for buildmaster@archlinux32.org

I believe, it means, that it may fall back to http1.1 - the documentation is not clear to me on this.
A simple test however shows, that at least curl has no problems to use http1.1 or http1.0 with the http2 enabled nginx.