Hi Samuele,
The problem occurs because when TabStrip2 is being initialized, MultiPage1 has not been initialized yet. So TabStrip2 was not able to see MultiPage1. Without a CallbackPanel, such initialization is triggered by the onload event, at which point all objects already exists. However this is not possible during an AJAX callback.
In order to get around this problem, you can put MultiPage1 inside a div:
Code: HTML/ASPX
<div runat="server" id="MultiPageContainer" style="display:none">
...MultiPage1 goes here...
</div>
In your server code, you can do:
Code: C#
MultiPageContainer.Style["display"] = "block";
or
Code: C#
MultiPageContainer.Style["display"] = "none";
To show/hide the MultiPage. Note at here you do not use the Visible property.
Thanks