Updating problems

I am having trouble editing. I found I got a ‘Page not found’ error after an edit when I tried to Update or Preview. Experiment showed that I can add plain text and a link to a file, but when I try to add a link to an item in the Media Library, or change such a link, I get the problem, although I can update and edit the same page when free of the <img..> directive.

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