Hi Enrico,
Dec 30 22:26:27 gissi imaplocal[17728]: IOERROR: opening /opt/iexpress/cyrusimap
/conf/mailboxes.db: Permission denied
Dec 30 22:26:27 gissi imaplocal[17728]: DBERROR: opening /opt/iexpress/cyrusimap
/conf/mailboxes.db: cyrusdb error
from the error messages, its apparent that you dont have permission to user Cyrus. Please double check all dir's permission.
You can follow this doc,
http://en.tldp.org/HOWTO/Cyrus-IMAP.html-Arun
"A ship in the harbor is safe, but that is not what ships are built for"