Controls collection cannot be modified because the control contains code blocks (i.e. )

Para resolver esse problema deve ser alterado os <%= %> para <%# %>

Entendendo como o código será executado

<% %> – Server side code
<%-- --%> – Server Side Comment
<%@ %> – Page Directive
<%= %> – Display value
<%: %> – HTML Encode
<%$ %> – Datasource Expression
<%# %> – Binding Expression

Fontes:
http://forums.asp.net/p/1172295/1965060.aspx
http://stackoverflow.com/questions/778952/the-controls-collection-cannot-be-modified-because-the-control-contains-code-bl
http://stackoverflow.com/questions/6365017/asp-net-server-side-code-block-explanations