Shared as in user-shared mailboxes. I think it was a mailclient (TB) problem. After subscribing from webmail (roundcube) I'm able to see them. I will do some more testing to see if I can reproduce.
Did you read the bottom of <http://wiki.dovecot.org/ACL>? Maybe the dovecot-acl-list file needed to be refreshed and somehow this happened.
