Can't connect to mail server anymore

Question

I never touched anything on the server which belongs to postfix and dovecot. But for several days I can't connect to my mailbox anymore.

/var/log/mail.log while connecting:

Sep 19 15:58:11 censored-hostname dovecot: auth: Error: BUG: Authentication client sent unknown handshake command: REQUEST?1008074753?14732?1?6068faccfcf40bafc9d4bff43c24e74f
Sep 19 15:58:11 censored-hostname dovecot: imap: Error: Authentication server didn't send valid SPID as expected: MECH^IPLAIN^Iplaintext
Sep 19 15:58:11 censored-hostname dovecot: imap: Error: Disconnected from auth server, aborting (client-pid=14732 client-id=1)
Sep 19 15:58:11 censored-hostname dovecot: imap-login: Internal login failure (pid=14732 id=1) (auth failed, 1 attempts): user=<[email protected]>, method=PLAIN, rip=censored-ip, lip=censored-ip, mpid=14733, TLS
Sep 19 15:58:11 censored-hostname dovecot: auth: Error: BUG: Authentication client sent unknown handshake command: REQUEST?3226730497?14734?1?007240b93b5d5f7c244d11409e9b03e7
Sep 19 15:58:11 censored-hostname dovecot: imap: Error: Authentication server didn't send valid SPID as expected: MECH^IPLAIN^Iplaintext
Sep 19 15:58:11 censored-hostname dovecot: imap: Error: Disconnected from auth server, aborting (client-pid=14734 client-id=1)
Sep 19 15:58:11 censored-hostname dovecot: imap-login: Internal login failure (pid=14734 id=1) (auth failed, 1 attempts): user=<[email protected]>, method=PLAIN, rip=censored-ip, lip=censored-ip, mpid=14735, TLS
Sep 19 15:58:12 censored-hostname dovecot: auth: Error: BUG: Authentication client sent unknown handshake command: REQUEST?2949644289?14737?1?52378726316ead0da1733e3e2c8bb2ce
Sep 19 15:58:12 censored-hostname dovecot: imap: Error: Authentication server didn't send valid SPID as expected: MECH^IPLAIN^Iplaintext
Sep 19 15:58:12 censored-hostname dovecot: imap: Error: Disconnected from auth server, aborting (client-pid=14737 client-id=1)
Sep 19 15:58:12 censored-hostname dovecot: imap-login: Internal login failure (pid=14737 id=1) (auth failed, 1 attempts): user=<[email protected]>, method=PLAIN, rip=censored-ip, lip=censored-ip, mpid=14738, TLS
Sep 19 15:58:13 censored-hostname dovecot: auth: Error: BUG: Authentication client sent unknown handshake command: REQUEST?901382145?14739?1?cd9ac7c93707db11956f55a290b50005
Sep 19 15:58:13 censored-hostname dovecot: imap: Error: Authentication server didn't send valid SPID as expected: MECH^IPLAIN^Iplaintext
Sep 19 15:58:13 censored-hostname dovecot: imap: Error: Disconnected from auth server, aborting (client-pid=14739 client-id=1)
Sep 19 15:58:13 censored-hostname dovecot: imap-login: Internal login failure (pid=14739 id=1) (auth failed, 1 attempts): user=<[email protected]>, method=PLAIN, rip=censored-ip, lip=censored-ip, mpid=14740, TLS

Dovecot configuration:

# dovecot -n
# 2.0.19: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-042stab092.3 i686 Ubuntu 12.04.5 LTS reiserfs
first_valid_uid = 150
last_valid_uid = 150
mail_location = maildir:/var/vmail/%d/%n
passdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
protocols = " imap"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
  }
  unix_listener auth-userdb {
    group = mail
    mode = 0660
    user = vmail
  }
}
ssl_cert = </etc/ssl/certs/dovecot.pem
ssl_key = </etc/ssl/private/dovecot.pem
userdb {
  args = uid=150 gid=8 home=/var/vmail/%d/%n allow_all_users=yes
  driver = static
}
protocol imap {
  imap_client_workarounds = tb-extra-mailbox-sep
}

What's wrong and how to fix? It's about 5 years ago when I configured the mail server.

Answer 1

According to some, this error can be cleared by manually restarting dovecot, and I understand that has worked for you.

That said, no machine that hasn't been patched for five years should be considered fit-for-purpose. It's definitely time for you to bring that box up-to-patch!