I viewed several old posts re. this error, seems like it was already fixed in version 5.3. I'm running v6.0. Can you take a look? Thanks.
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Editor1.SetSecurityTemplateGalleryPath(
String.Format("{0}{1}\HTMLTemplate", ConfigurationManager.AppSettings("TemplatePath"), Session("ClientID")))
Error source is on line 167 of InsertTemplate.aspx.
fs.VirtualRoot=CuteEditor.EditorUtility.ProcessWebPath(Context,null,secset.TemplateGalleryPath).TrimEnd('/')+"/";