Shared forms authentication between ASP.NET 1.1 and 2.0 stopped working after installing Security Update for Microsoft .NET Framework 2.0 SP2 and 3.5 SP1 on Windows Server 2003 and Windows XP x86 (KB2418241).
Received the following error in Application Event Viewer after authenticating in a v1.1 web page and then browsing to a v2.0 web page:
Event code: 4005
Event message: Forms authentication failed for the request. Reason: The ticket supplied was invalid.
Solution
As of 11/1/2010: Apply Hotfix KB2433751.
<http://code.msdn.microsoft.com/KB2433751>.
Note: Download NDP1.1sp1-KB2433751-X86.exe worked with Windows XP.
References
<http://support.microsoft.com/kb/2418241>.
<http://support.microsoft.com/kb/2433751>.
<http://code.msdn.microsoft.com/KB2433751>.
<http://forums.asp.net/t/1609592.aspx>.