502 – Web server received an invalid response

Hi, Whenever I log in to my website, I got this message as below.

However, upon refresh or second attempt, it will be alright. It happen to any other templetes that I have tried.
Is there anything I have to do to ensure smooth log to my website ?

The browser I am using are IE7, Chrome & Firefox Mozilla.

“502 – Web server received an invalid response while acting as a gateway or proxy server.
There is a problem with the page you are looking for, and it cannot be displayed. When the Web server (while acting as a gateway or proxy) contacted the upstream content server, it received an invalid response from the content server.”

Thank you.