Hi Benny,
Make a backup of your web.config file then add the following to the
section of your web.config file:
<add name="Telerik_Web_UI_DialogHandler_aspx" verb="*" preCondition="integratedMode" path="Telerik.Web.UI.DialogHandler.aspx" type="Telerik.Web.UI.DialogHandler" />
Please post back with an update.
Thanks!