In Outlook Express, this is what a user will typically see:
my.server.name
+- Inbox
+- Shared
+- UserA
+- SharedFolderA
+- UserB
+- SharedFolderB
If he clicks on "Shared", "UserA" or "UserB" he gets an error message: "Could not select 'foldername' on the IMAP server. You might try refreshing your folder list to synchronize with the IMAP server." and "Invalid mailbox".
Note: This is a purely cosmetical problem, the folders that can contain messages (SharedFolderA and B) should still be selectable, or you have a different problem than described in this article.