Given this is Microsoft IIS, I would venture you will have better luck finding answers to things on Microsoft Community on this one given IIS is their product. If things are working in Internet Explorer but not other browsers, my best hunch would Group Security Policy related issue at this point most likely causing issues.
The only other thing I could recommend to try would be to use the web browser Dev Tools option to trace the connection that way to see if you can get further information as to what is really going on with things, only thing of which I can think to try to get some further information on this one.
To help users find verified answers, please do not forget to use the "Accept as Solution" button on any post(s) that helped you resolve your problem or question. I now work a compressed schedule, Tuesday, Wednesday and Thursday, so those will be the days I will reply to any follow-up posts.