I have changed the name of my admin foldder and now I'm having a problem when signing in.

When I go to:

https://www.mywebsite.com/myadminpage/index

I'm correctly sent to the login page. I enter my username and password and click Enter,
but then I'm incorrectly sent to the 'Page Not Found' page, and the URL is missing the 'index', like this:

https://www.mywebsite.com/myadminpage

If I now manually add 'index' to the URL then I'm sent correctly to my admin home page.

Why is 'index' automatically removed from the URL when I'm trying to login?