"The Controls collection cannot be modified because the control contains code blocks"
First, start the code block with <%# instead of <%= :
<head id="head1" runat="server"> <title>My Page</title> <link href="css/common.css" rel="stylesheet" type="text/css" /> <script type="text/javascript" src="<%# ResolveUrl("~/javascript/leesUtils.js") %>"></script></head>This changes the code block from a Response.Write code block to a databinding expression.
Since <%# ... %> databinding expressions aren't code blocks, the CLR won't complain. Then in the code for the master page, you'd add the following:
protected void Page_Load(object sender, EventArgs e){ Page.Header.DataBind(); }
I just ran into this problem as well but found another solution.
I found that wrapping the code blocks with a asp:PlaceHolder-tag solves the problem.
<asp:PlaceHolder runat="server"> <meta name="ROBOTS" content="<%= this.ViewData["RobotsMeta"] %>" /></asp:PlaceHolder>(The CMS I'm using is inserting into the head-section from some code behind which restricted me from adding custom control blocks with various information like meta-tags etc so this is the only way it works for me.)
I can confirm that moving the javascript with <% %> tags from the head to the form tag fixes this error
http://italez.wordpress.com/2010/06/22/ajaxcontroltoolkit-calendarextender-e-strana-eccezione/