Code & steps to replicate the issue.
<telerik:RadButton runat="server" ID="RadButton1" Text="Target Button" AutoPostBack="true" EnableViewState="true" />
<br />
<br />
<telerik:RadButton runat="server" ID="RadButton2" Text="Toggle Enabled State and Do PostBack" OnClientClicked="toggleEnabledState" AutoPostBack="true" />
<script>
function toggleEnabledState(sender, args) {
var radButton1 = $find("<%= RadButton1.ClientID %>");
radButton1.set_enabled(!radButton1.get_enabled());
}
</script>