Weird behavior with Cyrus shared mailboxes

[aurelien]

hello,
I cannot subscribe to shared folders, even if they are visible in the folder list. When i check the checkbox, RC says "SUBSCRIBE: Subscribe: Mailbox does not exist"*
Cyrus imap log :

t 16 23:01:28 rei roundcube: [457E] S: A0002 OK [CAPABILITY IMAP4rev1 LITERAL+ ID ENABLE ACL RIGHTS=kxte QUOTA MAILBOX-REFERRALS NAMESPACE UIDPLUS NO_ATOMIC_RENAME UNSELECT CHILDREN MULTIAPPEND BINARY CATENATE CONDSTORE ESEARCH SORT SORT=MODSEQ SORT=DISPLAY THREAD=ORDEREDSUBJECT THREAD=REFERENCES ANNOTATEMORE LIST-EXTENDED WITHIN QRESYNC SCAN XLIST URLAUTH URLAUTH=BINARY LOGINDISABLED COMPRESS=DEFLATE IDLE] Success (no protection) SESSIONID=<cyrus-7562-1413493288-1>
Oct 16 23:01:28 rei roundcube: [457E] C: A0003 SUBSCRIBE share/admin-team
Oct 16 23:01:28 rei roundcube: [457E] S: A0003 NO Subscribe: Mailbox does not exist
Oct 16 23:01:28 rei roundcube: [457E] C: A0004 LOGOUT
Oct 16 23:01:28 rei roundcube: [457E] S: * BYE LOGOUT received
Oct 16 23:01:28 rei roundcube: [457E] S: A0004 OK Completed

By the way, the mailbox exists and mails are delivered to it. How can I track down that? I could provide more information about the setup.

Thanks,

Aurélien