IIS 404 on default document, direct link OK

My WS 2012 R2 IIS website just started to give error 404 on url https://mysite , i e invoking the default page default.aspx. (Real site name is valid!)

It seems that no default page at all works! I have tried all of them (*.htm, *.html) but only the direct url works. Giving https://mysite/default.aspx (or any other direct url) works beautifully as before. As do also http.

default.aspx is on the top of the default pages list. The W3c logs just says there is a 404 0 error on the / site

Enabling detailed error on the client page says nothing usable, just error code 0x80070002 which means "page not found".

Do you have any idea on why suddenly NO default page at all seems to work, but the direct urls do?

BR // Bo