This error is most likely caused by malformed data in the POST request. The control failed to deserialize its client state as a result. You should check if your page passes XHTML/HTML validation using either the W3C Validator
or a Firefox plug-in like this one
If your page doesn't have major issues like unclosed tags, etc. we'll need to see a live URL. Alternatively, you can record of the browsing session that reproduces the error using FiddlerCap
. Make sure you're not accessing the site through http://localhost, but by using the computer name instead, otherwise it won't capture the traffic.
I hope this helps.
the Telerik team
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