ajax:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true"
adding this attribute EnablePageMethods causes the whole site stop working and you will get this yellow error page with the following error:
Directory ‘C:\Inetpub\wwwroot\wss\VirtualDirectories\80\_catalogs\masterpage’ does not exist. Failed to start monitoring file changes.
The fix for this problem, is a bug and a known issue, just install this hotfix KB957661 on your sharepoint front server.
Hotfix download link :
Hope this helps :)