Proxy Error, Port 80 and Port 443

As I was trying to login in a certain site last weekend, I was experiencing an error which appear on my computer screen. Here it is;

"Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /login.

Reason: Error reading from remote server

Apache/2.2.8 (Ubuntu) mod_ssl/2.2.8 OpenSSL/0.9.8g Server at xxxxx.com Port 80"

I am not really an expert about this kind of error. I also tried reloading it a lot of times but still the same problem occurred. I can't go into the site. I also don't have any idea what a Port 80 and Port 443 are? Any expert out there who can share what these things are? Thanks in advance! Have a great Monday!