The info from your report iis a bit scarce; we would need more to replicate this and also the version of the software and the OS you are running.

Note that this specific keyserver is pretty limited in that it does not take part in the syncronisation of the keyserver network and that it returns only partial keys which gpg won't accept.

well, so far it`s win 10 with 3.1.11

and this also doesn`t work

C:\Users\Michael>gpg --keyserver https://keys.openpgp.org --search-keys mail@mhammerbacher.de
gpg: error searching keyserver: Server zeigt einen unbestimmten Fehler an
gpg: Suche auf dem Schlüsselserver fehlgeschlagen: Server zeigt einen unbestimmten Fehler an

C:\Users\Michael>gpg --keyserver hpks://keys.openpgp.org --search-keys mail@mhammerbacher.de
gpg: error searching keyserver: Kein Schlüsselserver verfügbar
gpg: Suche auf dem Schlüsselserver fehlgeschlagen: Kein Schlüsselserver verfügbar

I am wondering why this key server is recommended...

workaround:
edit gpg.conf and dirmgr.conf manually
rem proxy in dirmgr
insert https keyserver in gpg