As to modify the page of error of web-proxy

RouterOS General
1

As to modify the page of error of web-proxy

2

I think you cannot modify these type of pages.
You can use the following solution,

  • HotSpot with authorization, client will get HotSpot login page with required information before accessing public server.
  • use DST-NAT rules to redirect clients to the specific page, when client is trying to open specific server address.