Unfortunately HTMLEditor component of AJAX Control Toolkit is not compatible with the UpdatePanel. The problem is when we use accents with the Mozilla Firefox browser and HTMLEditor is inside an UpdatePanel. Letters that contain accents are left with an unknown character (so is stored in the database or even returned a PostBack).

Can be tested using Mozilla Firefox on the site of the ASP.NET AJAX Control Toolkit. 
Write a word with accents and go to "Submit Content": http://www.asp.net/AJAX/AjaxControlToolkit/Samples/HTMLEditor/HTMLEditor.aspx

As an alternative to this problem there are multiple component Rich Text Editors, some using jQuery and others not.
Queneeshas provided us a list of 10 components that can be viewed here: http://www.queness.com/post/212/10-jquery-and-non-jquery-javascript-rich-text-editors

Hopefully next release of the AJAX Control Toolkit, this inconsistency and others (like the ModalPopup Extender that already referenced in my blog) are resolved once and for all. This is because there are more updated versions prior to that do not have these problems, and with the passing of time some parts were coming into conflict.

If you know of any alternative or want to know at this problem, you can visit the topic I created the section of the AJAX Control Toolkit in ASP.NET forum: http://forums.asp.net/p/1548141/3848763.aspx