400 response of server GTM tagging URL

After you've set up your server Google Tag Manager container, you might want to check if tagging server URL works correctly. 

To do so, open your tagging server URL in the browser. You will see a 400 response code. This is correct behavior. 

To validate tagging server, add /healthy at the end of your URL and open it in a browser. If everything works correctly, you should see a reply ok.