“This Page has been modified since you opened it. You must open the page again. Refresh page.” error message

open the page in SharePoint designer and add the below script “ MSO_PageHashCode clearing script” :

<script language=”javascript” type=”text/javascript”>
if(document.getElementById(“MSO_PageHashCode”))
{
document.getElementById(“MSO_PageHashCode”).value=”";
}
</script>

Sometimes , You may face this issue when using any button (like asp:linkbutton or asp:button) for navigating to different URL (i.e: using the postback property) because , it’ll submit page.form , So in order to avoid this you should use normal <a href=”"></a> or use asp:hyperlink control.