Hi,
I am getting same error, I have checked bin directory, it all references to the ("Infragestics") .
aspx :-
<%@ Register Assembly="Infragistics.WebUI.UltraWebGrid, Version=11.1.20111.2020, Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb" Namespace="Infragistics.WebUI.UltraWebGrid" %>
license:-
Infragistics.WebUI.UltraWebGrid.UltraWebGrid, Infragistics.WebUI.UltraWebGrid, Version=11.1.20111.2020, Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb