I have a set of accordion controls within a tab container control. I get The "RegisterRequiresViewStateEncryption() method needs to be called before or during Page_PreRender" error when I try two open the same application in more than two windows or tabs. For instance I open the asp.net website in IE it works fine, but if I open the same website again in mozilla or another IE window while the first one is open I get the above error. Does somebody have a solution to correct this.