404 errors most commonly arise when one or more AWPCP pages are deleted, hidden, made private or moved.
First thing you should check is whether you have any AWPCP pages in the trash, or if they were made private. All pages should be public and published for AWPCP to work.
Second, your pages must all have unique names for them. AWPCP requires this, as the page name is used to generate a slug for the page. You can verify this under Classified Admin > Settings > Pages tab.
Finally, if you are missing some pages as a result of deleting them by accident, you can restore them easily under Classified Admin > Settings > Pages tab, by clicking the "Restore" button. You should make sure that the Trash is empty first, otherwise you might generate additional copies of existing pages with permalinks like "place-ad2" (where the number 2 indicates that you have 2 copies of "Place Ad" in your site.
More on this topic in the documentation for Classified Pages here