The Web Form Converter is not currently documented or supported. However, it might work if you upload the form to the server after doing the conversion. There’s a button to do that in the Web Form Converter window, and also a menu item in the Form window.
In that case, I’m going to fall back to “this is not currently documented or supported”.
Ok, I think I see where the problem might be. You must have instrumentation enabled for the GETWEBTEMPLATE procedure in _EnterpriseWebPublishLib (on your server). If you turn that off, I think you won’t encounter that problem. It looks like there’s a bug in the instrumentation code, but if instrumentation isn’t turned on, it should work.