I have tried this
<head id="Head1" runat="server">
<title>Back Office</title>
<link href="~/Styles/MasterPage.css" rel="stylesheet" type="text/css" />
<link href="Styles/custom-theme/jquery-ui-1.8.12.custom.css" rel="stylesheet" type="text/css" />
<script type="text/javascript" src="<%= Page.ResolveClientUrl("~/scripts/jquery-1.5.1.min.js") %>"></script>
<asp:ContentPlaceHolder ID="HeadContent" runat="server">
</asp:ContentPlaceHolder>
</head>
The error message is

I think too much, it is just find using this
sorry
It seems that, you are trying to add controls to page dynamically. In this case, you can use the below code