Slug/URL segment - /1 is wrongly saved, when trying to set / on rootpage.
If you create a sysFolder or a standard page for that matter as a container for multiple sites, then it is impossible to make the root page of these sites to have just / as slug. It always adds 1.
The reason seems to be, that when validating the slug, it also considers the slug of the parent container folder, even though the root page of the site is configured in the Sites module.
I conclude this, as if I go and manipulate the DB-record of the container folder and replace / with container in slug, then everything works as expected again.