Reproduced with latest official (2016.1.113) and internal (2016.2.128) in all browsers. Not reproducible with 2015 Q3 SP1.

Code to reproduce the problem:

    <asp:Button runat="server" ID="GoStep2" OnClick="GoStep2_Click" Text="Next" />
            <asp:Panel runat="server" ID="Panel1">
                <asp:Button Text="aspButton" runat="server" />
                <telerik:RadButton ID="Button1" Text="radButton" runat="server" />
            </asp:Panel>
 protected void Page_Load(object sender, EventArgs e)
    {
        Panel1.Enabled = false;
    }
 protected void GoStep2_Click(object sender, EventArgs e)
    {
        Panel1.Enabled = true;
    }


1 click the Next  button

3. When the page loads after the postback the RadButton remains disabled, unlike the asp:Button.