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after upgrading AJAX Control Toolkit from 4x to 7 the error message below was displayed on clicking Ènglish or French buttons the message below was displayed.
Solved by temporarily changing the id (html1) to HTML1, that is changed and back again:
StepsChanged Line 4: <html id="Html1" runat="server">
to Line 4: <html id="HTML1" runat="server">
Selected “clean” and “rebuild” and “rebuild.
then changed line 4 back to the original
Line 4: <html id="Html1" runat="server">
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