See how Jan-July and Aug-Dec are still directories underneath the Maildir
directory even though in the IMAP structure they are underneath 2005?
Anyways, I think you borked something with your copying over... What
command did you use? Your subfolders of "Mailing Lists" should appear
when you do a ls -al in Maildir if they got copied over at all. If they
didn't get copied but still were listed in courierimapsubscribed then
you'd get an error similar to what you're reporting...