Unknown server tag error

In the case of an error of "Unknown server tag" format regarding AjaxControlToolkit, you should add the first line of code marked in the figure to the web.config as follows:

<add tagPrefix="ajaxToolkit" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit"/>







With this trick, you not only solve your problem, but also you won't have to write

<%@ Register Assembly="AjaxControlToolkit" TagPrefix="cc1" Namespace="AjaxControlToolkit" %>

anymore to each page to use Ajax elements.