I would add a single form to your masterpage, this may be the cause of your error.
I would also remove all other form server controls from your user controls and pages.
Try these steps:
- Go to
Light.master
master page file and make sure that this is in there somewhere<form id="form1" runat="server">
and a closing tag, the id may be different. - Go to the following files
AssignmentTab.ascx
andAdministrationPage.aspx
and remove any<form id="form1" runat="server">
and closing tags</form>