I was stumped today when my ASP.Net site started creating this error
__doPostBack is not defined
I had no idea as to what was causing it.
After numerous searches and a lot of links later, I hit the target.
The reason it generated that error is simple, there are no controls on the page that are set to do a postback, so the postback function does not get sent to the client.
However, you can force the page to send the function to the client by one simple line in your page_load function
ClientScript.GetPostBackEventReference( <YOUR CONTROL NAME>, "")
In my case I have a repeater and hence
Hope this helps and saves you some trouble.