Central Firewall management unresponsive. Unable to load page. Check your network connection; 504 Gateway Time-out

Im currently managing and deleting built-in Host Objects, Web Policies and so on and creating my own stuff on a fresh install of XG136 HA on a 400Mbit cable WAN line from Sophos Central Management.

While working it occours often that the Web-Frontend is not responding and shows a spinning wheel for minutes

and showing errors like:

On WebAdmin in a red box on top right corner:

Unable to load page. Check your network connection

Or the Browser showing an error:

504 Gateway Time-out

URL (edited for privacy) is: fw-sso.0101.fw.prod.hydra.sophos.com/.../index.jsp

If you wait 5 minutes and refresh the page, you can start working again until it happens again after a random short time.

Other issues:

Page not loading correctly