Use Response.Redirect instead of Server.Transfer* when redirecting a user to the correct page
Server.Transfer* is not allowed in the same request as login is executed, instead you should use Response.Redirect to redirect the user to the correct page.
The reason is that the identity set for the user, after login has succeeded, is not transferred to the newly created thread the Server.Transfer* is executed in.
This applies from version 4.6.2 Hotfix 1.