Some pages cause 502 Bad Gateway errors

Some pages work:
https://punchkickinteractive.com/blog/
https://punchkickinteractive.com/blog/index.php/apple/pc-world-reports-that-ms-windows-vista-runs-fastest-on-a-mac-laptop/

Others don’t:
https://punchkickinteractive.com/blog/index.php/ryan-unger/
https://punchkickinteractive.com/blog/index.php/feed/
https://punchkickinteractive.com/blog/index.php/mobile-marketing/open-mouth-insert-foot/

I have no clue what is going on because “https://punchkickinteractive.com/blog/index.php/feed/” has been working for a while with no problems. Then, out of the blue things stop working. I have not installed any new plugins. It’s really weird. Any ideas, please advise.