I just upgraded a site to latest DNN 5.6.3 from 5.6.2 using DR 4.10.13v. When I upload an image, usually I get the preview automatically. Now the page errors with the following message on auto postback.

A critical error has occurred.Invalid postback or callback argument. Event validation is enabled using  in configuration or  in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation. 

Hi Kellyo,
We have been getting a small number of issues associated with DNN 5.6.3. What we have found is that there has been a change in the web.config going from 5.6.2 to 5.6.3 that needs adjusting. In previous versions of DNN the enableEventValidation was set to FALSE. In the 5.6.3 version it has been set to TRUE. Can you go in and test what happens if you set it back to FALSE and see if that corrects your problem. If it doesn't then set it back to what it was. Let me know if this works as we haven't tested this out fully.
Thanks

Hi Don, Thanks I made the change and now auto preview works again .. all seems good. Thank you!

Kelly