Just a note for anyone wanting to know why certain people see a error. If you visit using any proxy chances are you'll see the error. I have server level protection that blocks just about all TOR proxies as well as many others. If you visit with a normal connection you'll never get this error. As you can see there is a person that can't access the site from work. Reason is, while he is using a legitimate proxy set up by the IT person, the server still see's it as a proxy and therefor throws up an error.
So if you're getting a 503 error then drop the proxy and connect without it. There are legitimate proxies used by some ISP's. Those should not be a problem.