submenu not OK in IE

Hiya,
on http://www.latonatrainingen.nl the submenu-items of the bp-social-child theme do not show OK on some computers with IE7 and IE9. Items are invisible.
I deleted all the css changes I made in the menu's and de-activated cache plugin, but that does not seem to help.

Any insights are appreciated.

Have a nice evening and regards,
Ronad