Confirmation URL broken, if absolute URL or alias with leading slash is used

If you use an alias in the "On confirmation URL" field on the settings page.
The click on the confirmation URL breaks with an error message:

Auf der Website ist ein unvorhergesehener Fehler aufgetreten. Bitte versuchen Sie es später nochmal.

InvalidArgumentException: Cannot redirect to an empty URL. in Symfony\Component\HttpFoundation\RedirectResponse->setTargetUrl() (line 86 of /Users/username/Sites/project/vendor/symfony/http-foundation/RedirectResponse.php).

The confirmation is actually triggered and good, but the redirect to the page breaks.

It works ok if you use an alias without leading slash or a system path. In my case "Skip response form" is activated, but don't know if this matters.