I get this error recently, after I migrated my site to a new server and upgraded from drupal 6.20 to 6.22. Whenever I try to add an image node (or my other custom-defined node in which there is an image upload field) I end up with a "page not found" page. Previously created nodes are displayed without any issue.
All the installed modules are exactly as before and are updated to their latest version. I really appreciate your hints.