Using either Chrome or Firefox, both have simple built in Developer Tools,
open the problem page with a computer that can access the site.
In the top right corner open the browser menu
In Chrome: click More Tools > Developer Tools > Network
In Firefox: click Web Developer > Network
Right click anywhere on the screen and select "Inspect" or "Inspect Element"
Check "Preserve Log" (Chrome) or "Persist Logs" (Firefox)
Refresh the page
Now right click anywhere on the list of events that are popping up below
Click "Save all as HAR with content" (Chrome) or "Save All as HAR" (Firefox)