My SEO URLs return 404 after a move or PHP upgrade, what’s wrong?

OpenCart friendly URLs 404 when the rewrite chain breaks, commonly a missing RewriteBase in .htaccess, the SEO URL setting left off, missing seo_url rows after a migration, or $_SERVER['REQUEST_URI'] behaving differently on a new server/PHP version. We restore the rewrite block, fix the SEO PRO / SEF configuration, and re-seed any missing seo_url keywords so your indexed URLs resolve again.