You are probably getting HttpRequestValidationException
in your server, but because of the Ajax in your page you don't see the error. You could turn off the Ajax temporarily to see the errors in your page.
HttpRequestValidationException is caused by the internal form validation of the ASP itself, and it is not particular problem with the RadInput.
You could search for some solutions provided in the web, check this site for example:
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed