No submenu with IE9 theme HockeyTime

I have the theme Hockeytime, but under Internet Explorer my sub-submenu doesn't work, but under Safari, Mozilla and Google Chrome it works fine.

Do you know any solution for it. It works with the plugin Posts in category menu

http://tltc.drsbelettering.nl

thanks a lot

As you can see I set the menu, and in the frontend I see the first submenu, but not the second one of that submenu. As you can see in the settings it's not shown on the frontend. But this is only in Internet Explorer.

And with the theme Extremesports it works well. So it is in the theme and not in the plugin.

thanks