There has been a changed behavior (depending on from what version you are upgrading) regarding the formlogin and basic authentication. Please check the release notes, its should give you some more info regarding what has been changed and how you can fix it for your environment.
you think that is my problem:
Bug: 45832 QMS: Using NTLM Authentication with Custom or Alternate Login Page
We are issuing an update to address vulnerability in authentication when using NTLM authentication
with Custom or Alternate Login Page.
After implementation of this fix the end-user will need to login using a username in the form
DOMAIN\UserID , if NTLM is used with Custom or Alternate Login Page.
To implement this update and ensure that the combination of NTLM/custom login page/alternate login
page is correctly secured please ensure that the following are implemented:
-Install SSL if not already installed. The fix for this combination involves transmission of user
credentials using clear text via HTTP Form standard.
I must implement SSL vor communication intern?