[support] Validation error, please try again. If this error persists, please contact the site administrator.

Hello, In Drupal6, in /admin/build/block, when i save the page, i have this error : "Validation error, please try again. If this error persists, please contact the site administrator." In fact i have this error on several D6 themes but not Garland. So i don't know if it's a error theme or other thing. Do you have an idea ? Thanks, Julien.