Post error

Hi, I’ve recently put together a site for my sailing club, it’s a multi site setup. Everything seems to work ok bar a few little link niggles. The problem we are having is if you create a post and then publish it a message appears:

This page isn’t working portchestersc.org is currently unable to handle this request.
HTTP ERROR 500

Once you refresh the page it goes back to normal, it only happens on post publishing and if you log out of the site.

Any ideas? All help greatly appreciated

thanks

The page I need help with: [log in to see the link]