If you get this error message "Only one instance of a ScriptManager can be added to the page.", make sure you have a .aspx page set as "start page" before wasting a lot of valuable time searching for an answer. I unfortunately did waste the time to look up the error with no one knowing the cause.

At first glance, it appears that you put a "ScriptManager" control on two pages like master page as well as .aspx page. After seaching high and low, I figured out the problem. Cheers!