i have recently succesfully installed CMS MS 1.2.3 on my webspace and it's working like a charm

I have one problem though in creating my menus.
Whenever I go to the admin-panel to design the pages (I believe it's menu "Content" --> "Pages", it's in dutch with me),
If I click there on the arrow to collapse the items so I can see the submenus, they don't open.
Clicking on "Collapse all" doesn't do anything else either.
I tried this in FF 2.0 and IE7 both with no luck...
The menu I'm using is the default built-in "CSSMenu Left + 1 Column".
If any details are required, let me know...
Thanks in advance!