Webbrowser content was block because it was not signed by a valid security certificate

I developed a C# windows from which has a webbrowser. When I let the webbrowser browse a webpage using WebBrowser.Navigate(String) method, it show "Content was blocked because it was not signed by a valid certificate." on its content and web page cannot be loaded.

I use IE to browse the same web page, it works just fine.

Is there any hint what should I look into?

Thanks!

Ben