ipv4 imap not accepting connections in mixed ipv4/ipv6

[djk]

I have two installations of dovecot 2.18, apart from the necessary 
changes to allow them to replicate amongst themselves the configs are 
identical. Both are running Ubuntu 14.04 and are kept up to date. One is 
running 64bit and the other 32 bit (for historical reasons). Both have 
ipv4 and ipv6 addresses.

The 64 bit installation allows access on port 143 and 993 on both ipv4 
and ipv6.

The 32 bit installation does not allow access on 143 on ipv4, but allows 
ipv4/ipv6 on 993.

Both installations happily allow connections to any other ports that 
dovecot might be using on both ipv4/ipv6, as well as everything else 
that is running (on other ports) on those machines.

There are firewalls, but no warning messages. Switching firewalls off 
makes no difference.

Tcpdump shows the SYN packets arriving on the (correct) interface.

Any suggestions?

Dirk
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# 2.2.18: /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.8 (0c4ae064f307+)
# OS: Linux 3.13.0-61-generic i686 Ubuntu 14.04.3 LTS 
auth_verbose = yes
doveadm_password = %Db234A!&., at vc$
first_valid_uid = 2000
imapc_features = rfc822.size fetch-headers
imapc_host = post.tobit.co.uk
imapc_port = 993
imapc_ssl = imaps
imapc_ssl_verify = no
imapc_user = %n
lda_mailbox_autocreate = yes
login_greeting = IMAP ready.
mail_attachment_dir = %h/attachments
mail_attachment_min_size = 64 k
mail_location = mdbox:%h:INBOX=%h/mail
mail_plugins = " fts fts_lucene notify replication"
mail_prefetch_count = 20
mailbox_list_index = yes
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character vacation subaddress comparator-i;ascii-numeric relational regex imap4flags copy include variables body enotify environment mailbox date index ihave duplicate
mdbox_rotate_size = 10 M
namespace inbox {
  inbox = yes
  location = 
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix = 
}
passdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
plugin {
  fts = lucene
  fts_lucene = whitespace_chars=@.
  mail_replica = tcps:<other end of replicant>
  sieve = file:~/sieve;active=~/.dovecot.sieve
}
postmaster_address = postmaster at tobit.co.uk
protocols = " imap lmtp sieve sieve"
service aggregator {
  fifo_listener replication-notify-fifo {
    mode = 0666
    user = vmail
  }
  unix_listener replication-notify {
    mode = 0666
    user = vmail
  }
}
service auth {
  unix_listener exim-auth {
    group = Debian-exim
    mode = 0660
    user = Debian-exim
  }
}
service config {
  unix_listener config {
    mode = 0666
    user = vmail
  }
}
service doveadm {
  inet_listener {
    port = 23999
    ssl = yes
  }
}
service imap-login {
  inet_listener imap {
    port = 143
  }
  inet_listener imaps {
    port = 993
    ssl = yes
  }
}
service lmtp {
  inet_listener lmtp {
    address = 127.0.0.1
    port = <lmtp port>
  }
  unix_listener lmtp {
    mode = 0666
  }
  user = vmail
}
service managesieve-login {
  inet_listener sieve {
    port = <sieve port>
  }
  process_min_avail = 0
  service_count = 1
  vsz_limit = 64 M
}
service managesieve {
  process_limit = 100
}
service replicator {
  process_min_avail = 1
  unix_listener replicator-doveadm {
    mode = 0600
    user = vmail
  }
}
ssl_cert = <[key file]
ssl_client_ca_dir = /etc/ssl/certs
ssl_key = <[key file]
ssl_protocols = !SSLv3 !SSLv2
userdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
protocol lmtp {
  mail_plugins = sieve
}