Dovecot floods logs with “disconnected”-messages

Sometimes dovecot fills the logs with a couple of messages like the following every second:

dovecot: imap-login: Disconnected (no auth attempts in 0 secs): user=<>, rip=999.99.99.99, lip=888.88.88.88, TLS, session=< sLcaRpHX9ABREGnJ >

The session is a different string on every line and the user field is never filled. The messages keep getting logged even after turning off the potentially offending akonadi/kmail client which sits behind the logged rip. The only thing I can do is restarting dovecot upon which the logging stops.

Important to note:
– logging in and using dovecot works, with various clients and also with this akonadi/kmail client
– ssl = required, disable_plaintext_auth = yes

I really don't know why this happens, especially since logging in works (the dovecot wiki says that this message means that the client is not configured for TLS, which is obviously not the case). I appreciate any hints!

Best,
Kevin

PS: My issue looks similar to Force Dovecot not to log connect/disconnect messages, but that hasn't been answered yet—and I'd prefer to fix the cause more than to prevent logging.

dovecot -n:

# 2.1.7: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-16-pve x86_64 Ubuntu 12.10
auth_cache_size = 2 M
auth_cache_ttl = 1 weeks
auth_master_user_separator = *
dict {
  quota = pgsql:/etc/dovecot/dovecot-dict-sql.conf.ext
}
first_valid_gid = 8
first_valid_uid = 999
hostname = mail.example.com
last_valid_gid = 8
last_valid_uid = 999
lda_mailbox_autocreate = yes
lda_mailbox_autosubscribe = yes
login_greeting = Dovecot ready
mail_gid = mail
mail_home = /var/vmail/%d/%n
mail_location = maildir:~/mail
mail_uid = vmail
maildir_very_dirty_syncs = 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 ihave
namespace inbox {
  inbox = yes
  location =
  mailbox Drafts {
    auto = subscribe
    special_use = \Drafts
  }
  mailbox Sent {
    auto = subscribe
    special_use = \Sent
  }
  mailbox Spam {
    auto = subscribe
    special_use = \Junk
  }
  mailbox Trash {
    auto = subscribe
    special_use = \Trash
  }
  prefix =
}
passdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
plugin {
  sieve = ~/.dovecot.sieve
  sieve_default = /var/lib/dovecot/sieve/default.sieve
  sieve_dir = ~/sieve
}
postmaster_address = postmaster@example.com
protocols = imap sieve
quota_full_tempfail = yes
service auth {
  unix_listener /var/spool/postfix/private/dovecot-auth {
    group = postfix
    mode = 0660
    user = postfix
  }
}
service imap-login {
  inet_listener imap {
    port = 0
  }
  inet_listener imaps {
    port = 993
  }
  process_min_avail = 2
  service_count = 0
}
service managesieve-login {
  inet_listener sieve {
    port = 2000
  }
  service_count = 0
}
ssl = required
ssl_cert = </etc/ssl/example.com.pem
ssl_cipher_list = ALL:!LOW:!SSLv2:ALL:!aNULL:!ADH:!eNULL:!EXP:RC4+RSA:+HIGH:+MEDIUM
ssl_key = </etc/ssl/example.com.key
userdb {
  args = uid=999 gid=8
  driver = static
}
protocol lda {
  mail_plugins = " sieve"
}
protocol imap {
  mail_max_userip_connections = 30
}