Only for information to help other people with the same problem.
When I used the browser edge instead of Chrome the problem were eliminated. Changing the browser was a hint from the Navigraph support.
Calling the page SimBrief - Dispatch.
Button “Accept all cookies” and button “Sign in” pressed.
It appears: “Sign in to your Navigraph Account”. I filled in the form.
Afterwards I land again on the page SimBrief - Dispatch with “SimBrief by Navigraph”.
If I now press the “Sign In” button, the same screen with “Sign In” appears again and again on the same page (SimBrief - Dispatch).