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

Hello, In Admin > Blocks (i using the Amadou theme with Drupal 6) when i save changes (it's the the same thing if i save without any change), i have this error : "Validation error, please try again. If this error persists, please contact the site administrator." I can't assign blocks to the left sidebar (changes are not saved). So there is nothing in the report menu, i don't know how to resolve it. Do you know why ? Thanks.