The problem is the current IMAP implementation.
E.g. my current Cyrus IMAP has following capabilities:
ID QUOTA STARTTLS NAMESPACE AUTH=DIGEST-MD5 MULTIAPPEND IDLE UNSELECT SORT NO_ATOMIC_RENAME UIDPLUS THREAD=REFERENCES IMAP4 IMAP4REV1 X-NETSCAPE AUTH=CRAM-MD5 THREAD=ORDEREDSUBJECT ACL LITERAL+
and zimbra imap has only these capabilities:
ID QUOTA NAMESPACE SASL-IR AUTH=PLAIN LOGIN-REFERRALS UNSELECT LITERAL+ BINARY UIDPLUS CATENATE CHILDREN IMAP4REV1
IMHO the ACL Capability is necessary to allow other users to share folders.

Question to the developers:
Is it possible to compile the imapd with one or more other IMAP options -
or is it possible to use another imap-server ? (apt-get install cyrus-imapd)
Thomas.