but you cannot name a page INDEX…
which means you cannot have a default page in the folder.
No index.html allowed.
So… if you have a portfolio page…
you cannot have mydomain.com/portfolio (which defaults to mydomain.com/portfolio/index.htmll)
you have to use something like mydomain.com/portfolio.html (no folder)
or
mydomain.com/portfolio/view.html
This was a bug introduced around Jan / Feb 2016.
Prior to this… you could create an index page / slug.