This was a tricky one, spawned from when sebsel failed to discover the Webmention endpoint for one of Zegnat's posts. In that case, the Webmention endpoint was a relative URL and sebsel was sending a Webmention to a URL that was also an HTTP redirect.
The new test, webmention.rocks #23, instructs you to send a Webmention to a URL that is a redirect, and that page advertises a relative URL endpoint. You'll need to make sure that the URL you pass to your relative URL resolver is the final URL after following the redirect, rather than the one you start with.
So give it a shot! Test if your relative URL resolution code works properly!