PAT users can't access website, but NAT users can?

I'm a web developer who makes and maintains websites so I don't have extensive knowledge of networking. I work for a small company that uses a third party hosting service to host our website. One of our clients is a large company that uses Port Address Translation. Unfortunately, when they visit our website it brings up the generic server page that just has the hosting provider's logo and the client's IP address.

Now, no one has ever complained about this before so this is news to me. I've tried to Google the problem, but it doesn't seem to bring up anything related to this issue. Everyone in our office is able to access the website fine. I can access it from home and school with no problem. I called our hosting provider, and they told me they can access the website fine and it must be a problem with our client's PAT configuration.

Our client claims it's the hosting provider that's having issues. Our client's IT guy says that when using a static IP address to connect to the internet (using NAT), then our website comes up fine, but when switched back to PAT it fails.

Now, my question is, who is at fault here? Is it the hosting provider or our client?

Sounds like the client site is using something more than just vanilla PAT (probably a proxy or possibly some security/threat-management gateway). Regardless, the host header is likely being stripped out of the request, so your server has no idea what website they're trying to access.

I would imagine this is a common problem for the client, and is something they should fix. You can probably work around it by getting a dedicated IP for your site.