I'm saying in the url:
You don't specify the page. So the web server is redirecting to the default page. The issue is that it doesn't forward form posts as it redirects, so when your form also posts to the url without the default page specified, the form data is ignored and doesn't work.
That is why a solution of changing the form action so that the page is specifically specified in the url would likely solve the problem.
You will see now that the login page works when you go to:
but not when you go go:
If the default page isn't specified (in the browser, not in your hosting) then that is causing the issue.