This error occurs because javascript is trying to retrieve the z-index order of the sub menu, which IE8 returns the default value of auto. As a workaround edit the menu style sheet as follows. Under the folder "c:\dnn\portals\_default\skins\MinimalExtropy\css" open the "menu.css" file and add a z-index of 0 or 1 like the example below.

/*  SUB Menu Normal */
.main_dnnmenu_submenu
{
 z-index:1;
 border:1px solid #C0D6E5;
}

After saving your change don't forget to force IE to reload the style sheet by pressing CTRL+F5.