If user uses the back button and gets into a state where they are logged in but displaying the login screen the logout button will trigger validation which could be confusing for the user.
Added CausesValidation="false" to the logout button, also updated the login control to ensure that the login panel is hidden when a user is logged in and visit's the home page (default.aspx)
If user uses the back button and gets into a state where they are logged in but displaying the login screen the logout button will trigger validation which could be confusing for the user.