However, there are a few previously-created (pseudo-)subpages - e.g. User:WikiSysOp/ToDo - which are not converted to "proper" subpages automatically. They seem to be regarded simply as pages with a slash in their title (thus missing the automatic "breadcrumbs" link to the parent page). Is there any way I can fix that? Normally I'd delete the respective pages and re-create them from scratch - but since it's not possible to completely erase all records of an existing page (AFAIK), that's not an option here.
What happens if you move the article to a temp name and then move it back?