Can't create new content: "Page not found"

It used to work ok. The last new page I made was a week ago according to 'recent posts'. I can't think of anything I've changed in the last week, but now when I try to create any sort of new content, after I fill in the form and click the submit button there's a pause of about 5 seconds and then I get a smart Drupal-themed "page not found" message. The log doesn't show anything other than the page not found message.
The URL of the 'page not found' is the same as that of the content creatio form, which displayed ok.

I get this for events, pages, stories, images.. that's all I've tried. I've tried uninstalling modules I've put in recently to play with - adwords, image captions.

I have a database backup from April 21.. so I could try going back to that as not much has changed, but it'd be nice to know what's gone wrong.