Incompatible Atlas with cuteeditor/Ftb

When I was working on something recently, I applied it to Atlas on a page using cuteeditor. I didn't expect an error. The details are as follows:

The page is as follows,

Place the cuteeditor control, scriptmanager, updatepanel, and two dropdownlists. Set the triggers of updatepnael as follows:

CodeAs follows:

Protected void dropdownlistincluselectedindexchanged (Object sender, eventargs E)
{
Dropdownlist2.items. Add ("C ");
Dropdownlist2.items. Add ("D ");
}

The original page is as follows:

However, after the selectedindexchanged event of dropdownlist1 is triggered, the page is changed.

Sometimes the unknowerror error will be reported when you click the button. I tried to use Ftb to report such incompatibility. I don't know if you have encountered this problem and how to solve it.