Workaround - placing the following script after the ScriptManager: 
<script type="text/javascript">
    Telerik.Web.UI.RadGantt.prototype.postback = function (args) {
        var postbackFunction = this.get_postBackReference().replace("arguments",
                Sys.Serialization.JavaScriptSerializer.serialize(args).replace(/\\/g, "\\\\").replace(/'/g, "\\'"));

        eval(postbackFunction);
    }
</script>