Sys.ArgumentException: Value must not be null for Controls and Behaviors.
Parameter name: element
I figured it must be one of the ValidatorCalloutExtender controls so I disabled all of them and there was no error! I enabled them one-by-one and noticed that only the ValidatorCalloutExtenders associated with a RequiredFieldValidator attached to an FCKEditor control caused the error. Now, an FCKEditor control has nothing to do with the ValidatorCalloutExtender, but I do remember having to set EnableClientScript to false to get the validation working with the RequiredFieldValidator.