I created a webpage, based on MS Ajax 1.0 and the AjaxControlToolkit. On this page I used a button in an UpdatePanel which held a popup. Unfortunatelly a full postback occurred when I hit the button.
The next thing I did was running the source-code of the control-toolkit within VS.NET. Looking at the sample for the popupcontrolextender I found out the calendar-control was working properly.
So I placed a button in the same popup as the calendar. This gave the following result: the calendar worked fine, the button gave a full postback.
In a thread about this problem, David Anson gave me the answer to my problem. The button got the extra property UseSubmitBehavior="false" and everything worked fine.