Hello Muhammad,

Thank you for posting in our community.

I believe that the reason why the controls are not rendered in Design mode is that the version of your application differs form to one that you have locally installed on your machine. By design when installing the products all necessary dlls, including the ones needed for the designer are installed in your GAC. Having this in mind, please verify that the installed version is 20.1.20201.4.

If this is not the case these exceptions are expected. For the sake of the test, you can create a brand new project, open your toolbox, drag and drop any controls of your choice and switch to design mode. If the controls are correctly rendered you can check the version reference in your aspx page. For example:

<%@ Register assembly="Infragistics45.Web.v20.1, Version=20.1.20201.4, Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb" namespace="Infragistics.Web.UI.GridControls" tagprefix="ig" %>

If it is 20.1.20201. please let me know and we will decide how to proceed further.
Looking forward to hearing from you.