Infragistics WebHierarchicalDataGrid: “theForm is undefined”

I am using Infragistics WebHierarchicalDataGrid inside my webapplications sometimes. After I deployed one of those applications onto the customers web server (Windows Server 2003 with IIS 6) I got the error mentioned above. I was wondering why, because on my development machine (Windows 7 with installed IIS) everything works fine. Finally renaming the form from “form1” to “theForm” did the trick.

<form id="theForm" runat="server">