Created Unassigned: "Web page has expired" when back button is clicked [27667]

Hello,
I try to use CalendarExtender from AjaxControlToolkit on my Search page and I have troubles with the Back button in the browser. I try to describe situation:
1. User hits the Search button and postback is fired. The server returns some results.
2. The user clicks on a link on the Search page (simple html link)
3. The user uses Back button in the browser
4. The browser shows: "Web page has expired"

When I put on the page regular <asp:ScriptManager> instead of <ajaxToolkit:ToolkitScriptManager> everything works fine - user can go back in browser. (But of course I can't use the CalendarExtender)

Can anyone advice me?

Thnaks
Nikol