Mac unable to browse to any websites hosted at a single IP

I am unable to browse to any websites that are hosted on a single IP address - a reseller web hosting account that I manage, with a number of websites on it. This occurs in all browsers. Other websites are fine. The issue is just with websites hosted at this reseller account.

Errors:

Safari:

Safari can’t open the page XXXXXXXX because the server unexpectedly dropped the connection.
This sometimes occurs when the server is busy. Wait for a few minutes, and then try again.

Chrome:

No data received
ERR_EMPTY_RESPONSE
Unable to load the webpage because the server sent no data.

Firefox:

The connection was reset
The connection to the server was reset while the page was loading.

I am able to SSH, ping, traceroute, etc to the IP address. I can even log into the reseller hosting control panel (WHM) which runs on a different port - it is purely a web browsing issue on port 80.

Other machines at the same connection as me have no problems - just my machine.

I have tried many different things to resolve this:

• Using different DNS (Google, OpenDNS, etc.)
• Flushing DNS cache
• Clearing of cache in all browsers
• Used both WiFi and LAN connections

If I connect my machine to a personal hotspot on my phone, I am able to browse to these sites no problems. Switching back to my office network, and the issue is back. This may suggest it is the office connection being blocked, however:

• There are other machines on this network that can access the same sites perfectly fine
• There are no blocks at the web host for this IP

I have checked with the Hosting Provider, and they can see no issues at their end.

My machine:

Mac Yosemite 10.10.4 Broswers: Safari (8.0.7 (10600.7.12)), Chrome (44.0.2403.107 (64-bit)), Firefox (39.0)

So, I am putting this weird issue out there in the hope that somebody has some ideas on what may be the cause of the problem.

Looking forward to some ideas.

Thanks,

Nathan