After trying everything and a good night sleep I got enlightend....

The error was not in the service but in the keycloak settings for the api user. There was 2 things in place that hide the real error.

1. While also wrongly configured the first service used a cached version of the call and therefore worked.
2. In keycloak the web origin parameter for the client needs to be set correctly
3. Double check if you reload the page without cache. Regardles of settings I was only able to get an uncached page if I hit shift+F5 in the browser. The "do not use cache" checkbox in Browser DevTools did have no effect.