Segfault with error 4 in doveadm-server

[holger at dehnhardt.org]

I set up dovecot on two servers with replication enabled. The replication is over a vpn connection.
I get massive segfaults from doveadm-server every few minutes.

dmesg:
doveadm-server[30030]: segfault at 0 ip 0000557fa16d2e62 sp 00007ffdcaafec50 error 4 in doveadm-server[557fa16a3000+41000]
[486397.225636] Code: 40 03 4c 8d 3c c3 48 8d 44 24 18 48 89 44 24 08 0f 1f 84 00 00 00 00 00 49 8b bc 24 60 01 00 00 e8 e3 2a 00 00 48 89 44 24 18 <48> 8b 13 48 8d 35 c4 8d 00 00 48 89 c7 e8 2c 2b 00 00 31 c0 c6 45

mail.err
dovecot: doveadm: Fatal: master: service(doveadm): child 29507 killed with signal 11 (core dumps disabled)

I am absolutely unaware of how I can come to the cause of this error. Can anyone give me a hint?

-----------------------------
Configuration:
# 2.2.13: /etc/dovecot/dovecot.conf
# OS: Linux 5.1.14-xen x86_64 Debian 8.11 
auth_debug = yes
disable_plaintext_auth = no
doveadm_password = xxxx
doveadm_port = 4711
listen = *
mail_access_groups = vmail
mail_gid = vmail
mail_home = /var/vmail/%d/%n
mail_location = maildir:~/mail
mail_plugins = " notify replication"
mail_uid = vmail
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 Junk {
    auto = subscribe
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox Trash {
    auto = subscribe
    special_use = \Trash
  }
  prefix = 
}
passdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
plugin {
  mail_log_events = delete undelete expunge copy mailbox_delete mailbox_rename
  mail_log_fields = uid box msgid size
  mail_replica = tcp:backup-vpn.local:12345
  sieve = ~/.dovecot.sieve
  sieve_after = ~/sieve/after
  sieve_before = ~/sieve/before
  sieve_default = /var/vmail/%d/default.sieve
  sieve_dir = ~/sieve
}
protocols = " imap sieve pop3"
service aggregator {
  fifo_listener replication-notify-fifo {
    mode = 0666
    user = vmail
  }
  unix_listener replication-notify {
    mode = 0666
    user = vmail
  }
}
service auth {
  unix_listener auth-client {
    group = vmail
    mode = 0660
    user = vmail
  }
  unix_listener auth-userdb {
    mode = 0666
    user = vmail
  }
}
service config {
  unix_listener config {
    user = vmail
  }
}
service doveadm {
  client_limit = 1
  idle_kill = 0
  inet_listener {
    address = master-vpn.local
    port = 12345
  }
  process_limit = 0
  process_min_avail = 0
  user = vmail
}
service replicator {
  client_limit = 0
  drop_priv_before_exec = no
  idle_kill = 4294967295 secs
  process_limit = 1
  process_min_avail = 0
  service_count = 0
  unix_listener replicator-doveadm {
    group = vmail
    mode = 0666
    user = dovecot
  }
  vsz_limit = 8 G
}
ssl_cert = </etc/ssl/certs/xxx.cert
ssl_key = </etc/ssl/private/xxx.key
userdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
protocol sieve {
  managesieve_implementation_string = Dovecot Pigeonhole
  managesieve_max_line_length = 65536
}
protocol lda {
  mail_plugins = " notify replication acl sieve"
}
protocol imap {
  mail_plugins = " notify replication acl"
}