WebAuthnComponents DOMException raised

I am trying to run that library on a server behind an apache2 proxy.

So far I learned that the relying party gives an origin error as long as there is also a port number been added.
The apache2 proxy does that and since the library is coded to use the full URL:

    challenge =
      Wax.new_registration_challenge(
        attestation: attestation,
        origin: endpoint.url,
        rp_id: :auto,
        trusted_attestation_types: [:none, :basic]
      )

the registration fails.

While the registration of a key is working now I am getting another error on authentication…

Phoenix log:

[debug] HANDLE EVENT "authenticate" in MyAppWeb.AuthenticationLive
  Component: WebauthnComponents.AuthenticationComponent
  Parameters: %{"value" => ""}
[debug] Replied in 529µs
[debug] HANDLE EVENT "error" in MyAppWeb.AuthenticationLive
  Component: WebauthnComponents.AuthenticationComponent
  Parameters: %{"message" => "The operation failed for an unknown transient reason", "name" => "UnknownError", "stack" => ""}
[debug] Replied in 327µs
[warning] [unhandled_message: {MyAppWeb.AuthenticationLive, {:error, %{"message" => "The operation failed for an unknown transient reason", "name" => "UnknownError", "stack" => ""}}}]

Console log:

DOMException: The operation failed for an unknown transient reason [authentication_hook.js:82:14](https:/XXXX/deps/webauthn_components/priv/static/authentication_hook.js)
WebAuthn error: DOMException: The operation failed for an unknown transient reason

Help/hints/thoughts very much appreciated
Perhaps I am missing something simple and obvious