Forms based authentication not working between .Net 2.0 and .Net 4.0 application

Question

I have several web applications running on an Windows Server 2003 with IIS 6.0.

The applications are running under Asp.net 2.0.

Recently I have installed a

Answer 1

I had to go the long way and opened a support case with Microsoft.

As it turned out, the relevant security updates from Microsoft Security Bulletin MS11-100 were missing:

http://technet.microsoft.com/en-us/security/bulletin/ms11-100.
Choose your operatingsystem and install the updates for .Net 2.0 and 4.0.

This updates fixed forms-based authentication without reconfiguration of the involved web applications.