A workaround is to disable the embedded jQuery and use an older version (for example, 1.11.1 which is the previous version the suite used): https://docs.telerik.com/devtools/aspnet-ajax/controls/scriptmanager/disabling-the-embedded-jquery

Repro steps:

- Add a dropdownlist, attach server handler
- select item

Expected: postback

Actual: error

<asp:Label Text="" ID="Label1" runat="server" />
<telerik:RadDropDownList runat="server" ID="rddl1" OnSelectedIndexChanged="rddl1_SelectedIndexChanged" AutoPostBack="true">
    <Items>
        <telerik:DropDownListItem Text="first" />
        <telerik:DropDownListItem Text="second" />
        <telerik:DropDownListItem Text="third" />
    </Items>
</telerik:RadDropDownList>


    protected void rddl1_SelectedIndexChanged(object sender, Telerik.Web.UI.DropDownListEventArgs e)
    {
        Label1.Text = (sender as RadDropDownList).SelectedText;
    }