Web Forms :: Weird Characters Inserted In The URL Of The MenuItem Tab?

In my website I use a <asp:menu> control in the master page. And I use Web.sitemap file to define the menu items. And everything works fine.But every few days when I click on the menu item tabs to change to another page, strange characters are embedded in the URL like this:

- http://www.mydomain.com/(A(5wNJHLUjywEkAAAAZDhkNW....))/nextPage.aspx

(supposed to be just http://www.mydomain.com/nextPage.aspx)

By checking the following MSDN link I realize that these strange characters stands for anonymous user:
http://msdn.microsoft.com/en-us/library/aa479315.aspx

However, I already set the following in web.config:

- <forms cookieless="UseCookies" .... />
- <anonymousIdentification cookieless="UseCookies" enabled="false" />
- <sessionState cookieless="UseCookies"/>

but these strange characters are still inserted into the URL every few days.Once it happens, I need to restart the website, or restart or recycle the Application Pools in IIS, then it will become normal again.