Forms-Based Authentication problems after IE6 updates

I have setup our ASP.NET application in a Windows 2000 Advanced Server with IIS 5.0 and the latest version of the .NET Framework.

I use the Forms-based authentication (with cookies) for the login.

The problem I have is the following. After updating the IE6 browser with the latest updates from Microsoft in three different lap-tops running Windows XP Professional, there is no way to log in again to the application when we are connected to our internal network. When we write down our username and password, the login page comes again and there is no way to enter. Anyway, when we log in from outside the local network, we have no problem.

I’m sure the problem may be a silly configuration problem but I have been losing a lot of time with it and I don’t know what else to do. What am I doing wrong ??

Thanx in advance.

David