Some browsers (OmniWeb, iCab) don't preemptively send the authorization header and so they are authenticated as an anonymous user in the ZMI. The result is that the ZMI frames are displayed but the file manager is not. But I've found a way in Zope 2.4 to ask the browser to send the authorization header. Just edit line 139 of App/Management.py from manage_workspace__roles__=('Anonymous',) to manage_workspace__roles__=('Authenticated',). I don't know if there's any issue by changing this line...