Category - HtmlEditorExtender
Here Mudassar Ahmed Khan has explained with example and attached sample code, how to use and integrate the ASP.Net AJAX Control Toolkit HtmlEditorExtender Rich Text Editor in a website.
Here Mudassar Ahmed Khan has explained how to solve the error (exception) occurring when using the AJAX Control Toolkit’s HtmlEditorExtender control in ASP.Net.

Sanitizer provider is not configured in the web.config file. If you are using the HtmlEditorExtender with a public website then please configure a Sanitizer provider. Otherwise, set the EnableSanitization property to false.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.