Activiation URL empty
Figured it out by debugging getSiteUrl in UrlHelper.

Setting siteUrl like 'siteUrl' =&gt; strtolower((isset($_SERVER['HTTPS']) &amp;&amp; $_SERVER['HTTPS'] != 'off' ? 'https://' : 'http://') . $_SERVER['SERVER_NAME']) is fine for browser requests, but $_SERVER['SERVER_NAME'] isn't available when running a console command.

The scheme is set correctly and since we have addTrailingSlashesToUrls set to false, the result became http:/.

Fixed for now by hard-coding siteUrl in our general.php for each environment.