Is this a new error on a site that is already running in production or are you currently developing it? Reason I ask is if you have markup you're copying to Kentico and some of it includes a
<form> tag, this will cause this error. Having nested
<form> tags on an asp.net page is not allowed.
Secondly, if you have disabled viewstate from any webpart or zone which require viewstate you may receive this error. Look at it this way, if your webpart or control has a form on it with a submit button, you need to have viewstate enabled.