Ubuntu Dovecot 2.2.9 several Errors, Transaction log as well as BUG message

[Kein Name]

Hello List,

I am running an Ubuntu 14.04.5 LTS Server with Dovecot 2.2.9, only local
filesystems and authentication done via mysql backend.
Server is a Strato Virtual Server running
2.6.32-042stab120.11 #1 SMP Fri Mar 10 16:52:50 MSK 2017 i686 i686 i686
GNU/Linux

Sometimes I see errors in the log like this for one user:

Error: Transaction log /var/vmail/
example.net/jena/Maildir/.Archives.2017/dovecot.index.log: duplicate
transaction log sequence (2)

After seeing these, some time later, (mostly 2h), Dovecot starts to deny
login from all clients with messages like this:

Mar 23 01:32:27 h2312250 dovecot: auth: Error: BUG: Authentication client
sent unknown handshake command:
REQUEST?1804861441?19318?1?9ce500129289a0237e4e85a6c880a047?session_pid=19323...
Mar 23 01:32:27 h2312250 dovecot: imap: Error: Authentication server didn't
send valid SPID as expected: MECH#011PLAIN#011plaintext
Mar 23 01:32:27 h2312250 dovecot: imap: Error: Disconnected from auth
server, aborting (client-pid=19318 client-id=1)
Mar 23 01:32:27 h2312250 dovecot: imap-login: Internal login failure
(pid=19318 id=1) (internal failure, 1 successful auths): user=<
user at example.net>, method=PLAIN, rip=X, lip=X, mpid=19323,
session=<G9f4AltL4ABRqZ4X>

I do not know if these problems have something to do with each other or
not. But it happened several times now (maybe once or twice every month or
so).
When I restart Dovecot, the error seems gone for the moment, until it comes
back in a month or some weeks.


For reference, here is the running config:

dovecot.conf

# 2.2.9: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-042stab120.11 i686 Ubuntu 14.04.5 LTS
auth_mechanisms = plain login
auth_verbose = yes
disable_plaintext_auth = no
log_timestamp = "%Y-%m-%d %H:%M:%S "
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
passdb {
  args = /etc/dovecot/dovecot-mysql.conf
  driver = sql
}
plugin {
  quota = maildir:User quota
  quota_rule = *:bytes=10M
  sieve = ~/dovecot.sieve
}
protocols = imap pop3 sieve
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0660
    user = postfix
  }
  unix_listener auth-master {
    mode = 0600
    user = vmail
  }
  user = root
}
ssl_cert = </etc/postfix/sslcert/mailserver.crt
ssl_key = </etc/postfix/sslcert/mailserver.key
userdb {
  args = /etc/dovecot/dovecot-mysql.conf
  driver = sql
}
protocol pop3 {
  mail_plugins = quota
  pop3_uidl_format = %08Xu%08Xv
}
protocol imap {
  mail_plugins = quota imap_quota
}
protocol lda {
  auth_socket_path = /var/run/dovecot/auth-master
  mail_plugins = sieve quota
  postmaster_address = postmaster at example.net
  quota_full_tempfail = yes
}


dovecot-mysql.conf

driver = mysql
connect = host=localhost dbname=mail user=user password=XXX
default_pass_scheme = CRYPT
password_query = SELECT password FROM users WHERE email = '%u'
user_query = SELECT
CONCAT('/var/vmail/',SUBSTRING_INDEX(email,'@',-1),'/',SUBSTRING_INDEX(email,'@',1),'/')
AS home,
CONCAT('maildir:/var/vmail/',SUBSTRING_INDEX(email,'@',-1),'/',SUBSTRING_INDEX(email,'@',1),'/Maildir/')
AS mail, 5000 AS uid, 5000 AS gid, concat('*:bytes=', quota) AS quota_rule
FROM users WHERE email = '%u'


Any help is appreciated.
Thanks
Stefan Koenig