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You are trying to add controls to your page but you are unable to drop controls into your content place holder.
The Content Place Holder must be on one line not split on to two lines in your master page.
Open the master page up in a text editor and make sure that the content place holders are on one line like this:
<asp:ContentPlaceHolder ID="LeftContentPlaceHolder" runat="server"></asp:ContentPlaceHolder>
not split onto two or more lines like this:
<asp:ContentPlaceHolder ID="LeftContentPlaceHolder" runat="server">
</asp:ContentPlaceHolder>