ok thanks @jangaraj for all your help and advises. I am writing as a reference for others:
I entered there URL because I read something like “KC_HOSTNAME” is the replacement of “KEYCLOAK_FRONTEND_URL” variable in somewhere.
Of course it’s not hard to test it. I tried it as “keycloak.mydomain.com” and also as “keycloak”; nothing changed.
Maybe that’s the same problem:
Thanks & Regards