dovecot - Nov 2013 - Dovecot panic during replication

I'm seeing the following errors on my Dovecot box when trying to replicate:

Nov 15 18:44:46 server1 dovecot: dsync-server(mark): Panic: file
mbox-lock.c: line 799 (mbox_lock): assertion failed: (lock_type == F_RDLCK
|| mbox->mbox_lock_type != F_RDLCK)
Nov 15 18:44:46 server1 dovecot: dsync-server(mark): Fatal: master:
service(doveadm): child 21571 killed with signal 6 (core not dumped - set
service doveadm { drop_priv_before_exec=yes })


And on the target I see:

Nov 15 17:47:02 server2 dovecot: dsync-server(mark): Error: read(
server1.mdb.eu.com) failed: EOF


Does anyone know what is causing this?  I help to solve this before I can
put it live :-(

Here's my dovecot -n from server1:

# 2.2.7: /usr/pkg/etc/dovecot/dovecot.conf
# OS: NetBSD 5.2 cobalt
auth_mechanisms = plain login
doveadm_password = secret
doveadm_port = 12345
listen = 192.168.1.1
mail_location = mbox:~/mail:INBOX=/var/mail/%u
mail_plugins = " notify replication"
passdb {
  driver = passwd
}
plugin {
  mail_replica = tcp:server2.mdb.eu.com
}
protocols = imap
service aggregator {
  fifo_listener replication-notify-fifo {
    mode = 0600
    user = vmail
  }
  unix_listener replication-notify {
    mode = 0600
    user = vmail
  }
}
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0660
    user = postfix
  }
  user = root
}
service doveadm {
  inet_listener {
    port = 12345
  }
}
service imap-login {
  inet_listener imap {
    port = 0
  }
}
service replicator {
  process_min_avail = 1
  unix_listener replicator-doveadm {
    mode = 0600
  }
}
ssl_cert = </etc/openssl/certs/mail.mdb.eu.com.crt
ssl_key = </etc/openssl/private/mail.mdb.eu.com.key
userdb {
  driver = passwd
}
protocol lda {
  postmaster_address = postmaster at example.com
}



Thanks.