Webadmin service should have a dynamic definition that should change if you change it in management-->webadmin settings.
Screenshot 1: Webadmin service has a static definition that opens port 4444 .
Screenshot 2: Webadmin is also added by default to advanced tab of http proxy under allowed target services.
Screenshot3: If you manually change the webadmin port in management-->webadmin settings, the original definition is not changed.
Therefore if you are using standard proxy, you are locked out of webadmin till you find a way to go back in astaro and change the webadmin definition. Here are the screenshots, hope I was clear enough.