zielinski

You become some 404 errors. What is in the server logs?

That is my walkaround:

add_filter('webpc_htaccess_mod_rewrite', function($rules) {
  return '<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{HTTP_ACCEPT} image/webp
    RewriteCond ' . WP_CONTENT_DIR . '/uploads-webpc/$1.$2.webp -f
    RewriteRule (.+)\.(jpe?g|png)$ /wp-content/uploads-webpc/$1.$2.webp [NC,T=image/webp,E=cache-control:private,L]
  </IfModule>';
});

You can minimize you .htaccess by putting the extension to the second regex group.