Issues making website browser friendly

Hi,

My team created an login screen as href whenever you try to get into the website (Redirecting from all URL's) but we have an issue...
Testing this on Internet Explorer, we found out it's impossible to get into the website due to an 403 Forbidden error.

Going directly into the login page (by adding login.html), we get into the webpage but as soon as we try logging in, same error...

Is this a known issue? If yes, I'd really like to know what you have done to fix this.

Thank you in advance.