my theme breaks when I go to edit/add classified

I have set this page up just to see what the shortcodes do.
All pages come up fine but when I click on Add/Edit classified my theme breaks.
It is using 'page.php' to show these items. This theme file seems to be ok for all the other pages but not for this one.
Not sure what I have done wrong here or where to go to fix it.
Any help would be appreciated.

Many thanks