I have a page that is supposed to be a subpage of another page, so in the CMS I go to that subpage and in Page Attributes I choose that other page as parent and click Update.
As soon as I try to test that new page, I get a “nothing found” page.
The URL is correct, the permalink in CMS is correct, I’ve flushed permalinks, but nothing helps.
As soon as I set it to “no parent” again, the problem is solved.
The weirdest thing is that I have other pages, directly from the theme, with parent pages that work fine.