We have a simple web page which displays a treeview ( Microsoft.Web.UI.WebControls )

Now, on some machines (mainly the clients ones ;-) ) the treeview isn't rendered properly :

I've tested this on all my machines, even linux, and everything is rendered correctly here. I can reproduce this error though by removing or renaming the 'webctrl_client' folder.
I don't understand however what this problem can be? The 'webctrl_client' folder is correctly installed in the root www folder, and the .dll is also properly placed? I tried clearing the browsers cache but that doesn't seem to have any effect either ....
Doen anyone have any idea what could be going on?