The most probable cause for this error is that the web application was recycled. RadSpell keeps its parameters in encrypted form and the encryption key is unique and changed each time the web application starts. If you open the page with RadSpell and then for some reason recycle the web application, the key will be changed and you will see the "Cannot deserialize dialog parameters" error when trying to use the spell control. To fix this you need to refresh the page and make sure that your application is not restarted too often.
If you cannot stop the application from restarting, you can try to workaround the problem by choosing a fixed key for encryption of the spell parameters. See the following help article for more details - http://www.telerik.com/help/aspnet-ajax/editorcannotdeserializedialogparameters.html
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