Authentication cookie lost

I also have some reference information for you.

1. I also found post that say the problem appeared after “reformat” the server. It looks similar to our case. Maybe can give you a hint.
http://www.velocityreviews.com/forums/t81921-forms-auth-cookie-vanishes-immediately-after-login.html

How about just remove the site and create new one with different name/description? Sometime it works.

2. The problem does not appear on FireFox that means there should be no problem with the connection between Web server and SQLServer session. It means the problem is the Authentication Cookie lost problem. We need to find out where the cookies lost (server/client).

Here is the tool (free) that can help to check the header information (include cookie) sent between browser and web server.
http://www.fiddler2.com/Fiddler2/version.asp

Here is the Microsoft Help and Desk information on cookie lost.
http://support.microsoft.com/kb/910439

3. I also checked related source code in Australia project, the only suspicious place is the “User” property of UIBase. But I don’t think it will affect “HttpContext.Current.User”. If you think it is necessary, I can change it to use something else like “Actor”. It will involve changes in almost all pages in Web project.