After today's rawhide update SM displays $default_folder_prefix = ''; on top of the login page and only $default_folder_prefix = ''; once you're logged in
Created attachment 132223 [details] Screenshot
-> FC6Target (maybe even Blocker ?)
The same problem is caused by the latest update for Fedora Core 4 (squirrelmail-1.4.7-2.fc4). Also see bug 198334 - this one seems to be a dupe of that one. There's the diagnosis and a fix over there.
*** This bug has been marked as a duplicate of 198306 ***