The reason for this error is that your having more than one FORM tag in your aspx page with run at server attribute, which ASP.Net does not allow. You can have only one form tag in your aspx page with run at server attribute. So keep only one form tag in user aspx page to avoid this error.
If you run below aspx page you will get same error as it got two server form tag.
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
</div>
</form>
<form id="form2" runat="server">
</form>
</body>
</html>