Hi Xorcist,
The problem is caused by security restrictions in the ASP.NET framework and is not directly related to RadTextBox. Please refer to:
http://www.asp.net/learn/whitepapers/request-validation/
Since there is no good and proper way to intercept postbacks and change submitted values on the fly, I can suggest you to use some custom client event (e.g. a button click) and encode values before the actual submit action.
If you prefer using more hackish methods, here are two for you:
1. Override the
WebForm_OnSubmit function, which handles all postbacks. For example
http://www.duncangunn.me.uk/dasblog/2008/03/07/InterceptingWebFormOnSubmit.aspx
2. Change the value of the request body on the fly:
Greetings,
Dimo
the Telerik team