Update: In the last release of AjaxControlToolkit, the problem had been solved
I never used the Accordion control before. Last days, I had to work on another developer's code that used an Accordion control. One of the problems the code had was that when you click on a button that inside the Accordion control - the first click doesn't do nothing, only the second click fire the event. The control was inside an updatepanel, so I was sure that make the problem, but it still happened after I took it out from the updatepanel.
I searched the Internet, and I found that is very common problem with that control. I tried lot of solutions suggested, like adding a reference to the button in the OnInit page event, adding manually the event, and some stuff similar, but nothing really worked. After I almost got crazy, I found this post and it had some source code that claimed to be the solution. I downloaded the whole source of the AjaxControlToolkit.dll, change the code in the Accordion class with the attached code in the post, compiled it, replaced the AjaxControlToolkit.dll in my project with the new compiled one - and Walla!!! the Accordion starts to work as expected, and to fire all the needed events!
So, if you have that problem, that events are not being fired from withing your Accordion control, and you are lazy to download and compile the source code of the AjaxControlToolkit, just download the file below, replace the AjaxControlToolkit.dll in your project with this one, and your Accordion will work just fine.
AjaxControlToolkit.zip (402.69 kb)