Microsoft Ajax throwing error

I have just started working on an asp.net web application.
I am using bootstrap for UI part.

On one of the page I have used a asp.net drop down which shows user types in it.
And a repeater control to show users of the selected user type.
User can user type from the drop down and below repeater will refresh and show users.
This page was working fine till the time I added updated panel in to it.
I added repeater inside the update panel.

And in the page load I have screen code for the dropdown

ScriptManager scriptManager = ScriptManager.GetCurrent(this.Page);

scriptManager.RegisterAsyncPostBackControl(ddlUserTypes);

and this is the drop down index change event

  protected void ddlUserTypes_SelectedIndexChanged(object sender, EventArgs e)
        {
            PopulateUsers();
            this.upUsers.Update();
        }

Now when I run the page and select a new user type from the drop down I am getting this error (attached the image).
I am using visual studio 2012.
I googled but I did not find the any workable solution for this and my client is sitting on my head.

Can any one help me out here please.