Hi Luca,
Please, try the forth suggestion in this KB article:
Error: Web.config registration missing! The Telerik dialogs require a HttpHandler registration in the web.config file, e.g.
- open the web.config file and replace Telerik.Web.UI.DialogHandler.aspx with Telerik.Web.UI.DialogHandler.axd
- set the RadEditor's DialogHandlerUrl property to "~/Telerik.Web.UI.DialogHandler.axd"
All the best,
Rumen
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