I am seeing this;

Jun 07 16:17:22 Warning: lmtp(nino@sidoti.id.au)<20211><vJkREfC5vWDzTgAAv/fbcw>: replication(nino@sidoti.id.au): Sync failure: Timeout in 2 secs
Jun 07 16:17:22 Panic: replicator: data stack: Out of memory when allocating 268435496 bytes
Jun 07 16:17:22 Fatal: replicator: master: service(replicator): child 77583 killed with signal 6 (core not dumped - https://dovecot.org/bugreport.html#coredumps)

It started when I added the following to my configuration;

replication_sync_timeout = 2

If I remove the above line then sync works properly and no errors. 

# 2.3.14 (cee3cbc0d): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.5.14 (1b5c82b2)
# OS: OpenBSD 6.9 amd64  ffs
# Hostname: obsd-mx1.stonyrange.com
doveadm_password = # hidden, use -P to show it
doveadm_port = 2727
hostname = mx1.stonyrange.com
listen = *
mail_gid = vmail
mail_home = /var/vmail/%d/%n/Home
mail_location = maildir:/var/vmail/%d/%n/Maildir
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 index ihave duplicate mime foreverypart extracttext imapsieve vnd.dovecot.imapsieve
namespace inbox {
  inbox = yes
  location = 
  mailbox "Deleted Messages" {
    special_use = \Trash
  }
  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 = scheme=BLF-CRYPT /etc/mail/passwd
  driver = passwd-file
}
plugin {
  imapsieve_mailbox1_before = file:/usr/local/lib/dovecot/sieve/report-spam.sieve
  imapsieve_mailbox1_causes = COPY
  imapsieve_mailbox1_name = Junk
  imapsieve_mailbox2_before = file:/usr/local/lib/dovecot/sieve/report-ham.sieve
  imapsieve_mailbox2_causes = COPY
  imapsieve_mailbox2_from = Junk
  imapsieve_mailbox2_name = *
  mail_replica = tcp:108.160.141.103:2727
  replication_sync_timeout = 2
  sieve = file:~/sieve;active=~/.dovecot.sieve
  sieve_global_extensions = +vnd.dovecot.pipe +vnd.dovecot.environment
  sieve_pipe_bin_dir = /usr/local/lib/dovecot/sieve
  sieve_plugins = sieve_imapsieve sieve_extprograms
}
protocols = imap lmtp
service aggregator {
  fifo_listener replication-notify-fifo {
    user = vmail
  }
  unix_listener replication-notify {
    user = vmail
  }
}
service auth {
  unix_listener auth-userdb {
    user = vmail
  }
}
service doveadm {
  inet_listener {
    port = 2727
  }
  user = vmail
}
service imap-login {
  inet_listener imap {
    port = 0
  }
  inet_listener imaps {
    port = 993
    ssl = yes
  }
}
service managesieve-login {
  inet_listener sieve {
    port = 4190
  }
}
service replicator {
  drop_priv_before_exec = yes
  process_min_avail = 1
  unix_listener replicator-doveadm {
    mode = 0600
    user = vmail
  }
}
ssl = required
ssl_cert = </etc/ssl/stonyrange.com.fullchain.pem
ssl_dh = # hidden, use -P to show it
ssl_key = # hidden, use -P to show it
userdb {
  args = /etc/mail/passwd
  driver = passwd-file
}
protocol imap {
  mail_plugins = " notify replication imap_sieve"
}
protocol lmtp {
  mail_plugins = " notify replication sieve"
}