Error message "Only one instance of a ScriptManager can be added to the page." when trying to access my web application. Compiled have no error.
The reason for this error is i have include more than one (my case is two) ScriptManager in the same page. In my case in master page and in another page (login.aspx) that include master page.
To solve it, simple remove the ScriptManager in login.aspx. Problem solved.