what's your SSL certificate situation on the server. Anything?
With other browsers asking for authentication I think you should look into identifying what is causing that to pop up.
Also, since that's a 192.168 IP it seems off to me. Is that the browser's IP listed in the filemaker control panel under the "web server" section? Sometimes that may have a different IP address depending on deployment.
A blank page is still something being returned, have you used the dev tools in those browsers to see if any error messages are displayed in any of the tabs tracked?
Lastly, it may serve well to read through the 15 guide and see if there are any steps you may have missed: