Hi!

in mailbox_autocreate:

if (mailbox_create(box, NULL, FALSE) < 0) {
    errstr = mailbox_get_last_error(box, &error);
    if (error != MAIL_ERROR_NOTFOUND) {             <<< May be MAIL_ERROR_EXISTS should be here?
        mail_storage_set_critical(box->storage,
            "Failed to autocreate mailbox %s: %s",
            box->vname, errstr);
        return -1;
    }

Some time I got this error when two connections tries to create INBOX at same time.

С уважением, Сергей Потапов