User unable to submit blog ' Forbidden access'

Hi

one of my bloggers is unable to submit posts, his username is joe ponting - heres his profile page http://www.blatantlyblunt.com/Blog/Blogger/Listings/joeponting.html

Whenever he submits anything, e.g. just text and a youtube embed, it goes straight to a plain white page that just says Forbidden access across the top

other bloggers are not having this issue, any idea what may be causing it?

thanks