Hi!
I need more information on the “domain.com/undefined” issue.
- Since when is it happening? Have you changed any configuration regarding result URL?
- Is it happening on custom post types or regular posts or something else?
- Is it happening on every result, or only some of them?
- Do you have a working url where I can check this?
- Are you using any kind of filter function on the results?
The result URLs are parsed via the built-in get_permalink( $id, $leavename ) WP core function, so if this is returning an incorrect url, then this might be related to something else as well.