Link to FAQ broken (SAML)

Discourse instance with version 2.8.10, SAML (keycloak) works fine but found one major issue:

  • Link from external app (e.g. in Gmail on another browser tab) to /faq forwards to /login and doesn’t display FAQ (fail)
  • Link from internal app (e.g Wiki which is connected via keycloak/saml, too) to /faq works. (OK)
  • Same URL (/faq) if I type it in the browser URL field works. (OK)
  • Link (e.g. in mail) to a specific topic or category works (OK)

Someone has a clue whats going on here?

Nobody has a clue about this issue?