Do you use Cyrus?
I encounter almost the same problem. After I created shared mailboxes with cyradm, the only way my users can use them is by giving them at least admin rights. When I want to access the folders in Thunderbird, the admin tag is not needed.
Can somebody explain me why a user needs admin rights to be able to see/use a shared folder in OX?
Some extra info. When I tag a folder for a user with lrswa, the user can see the folder in OX and can use it. But he isn't admin. By the time the user also gets the k tag, he suddenly becomes admin, but is not able to create sub-folders (not in OX, in Thundebird hen can).
This is what man cyradm tells me about the tags:
l Lookup (mailbox is visible to LIST/LSUB, SUBSCRIBE mailbox)
r Read (SELECT/EXAMINE the mailbox, perform STATUS)
s Seen (set/clear \SEEN flag via STORE, also set \SEEN flag during
w Write flags other than \SEEN and \DELETED
i Insert (APPEND, COPY destination)
p Post (send mail to mailbox)
k Create mailbox (CREATE new sub-mailboxes, parent for new mailbox in RENAME)
x Delete mailbox (DELETE mailbox, old mailbox name in RENAME)
t Delete messages (set/clear \DELETED flag via STORE, also set \DELETED
flag during APPEND/COPY)
e Perform EXPUNGE and expunge as part of CLOSE
a Administer (SETACL/DELETEACL/GETACL/LISTRIGHTS)