A 50X error is an internal server error. There's nothing wrong on your end, but something's up on the server's end.
http://www.checkupdown.com/status/E503.html
The Web server (running the Web site) is currently unable to handle the HTTP request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay.
Just be patient and wait. :-)