assertion failed: (srcleft <= CHARSET_MAX_PENDING_BUF_SIZE)

[Giacomo]

I'm running the following:

# 2.3.4 (0ecbaf23d): /usr/local/etc/dovecot/dovecot.conf
# OS: FreeBSD 11.2-RELEASE-p4 amd64
# FS: ZFS

auth_mechanisms = plain login
auth_username_format = %Ln
listen = *
mail_location = maildir:~/Maildir
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 {
  driver = pam
}
protocols = imap lmtp
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0666
    user = postfix
  }
  user = dovecot
}
service imap-login {
  inet_listener imap {
    port = 0
  }
}
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}
service stats {
  unix_listener stats-reader {
    group =
    mode = 0600
    user =
  }
  unix_listener stats-writer {
    group = $default_internal_group
    mode = 0666
    user =
  }
  user = $default_internal_user
}
ssl_cert = </root/cert.pem
ssl_key = # hidden, use -P to show it
userdb {
  driver = passwd
}

dmesg shows:

pid 74441 (imap), uid 1002: exited on signal 6
pid 74784 (imap), uid 1002: exited on signal 6
pid 21471 (imap), uid 1002: exited on signal 6
pid 21470 (imap), uid 1002: exited on signal 6
pid 21464 (imap), uid 1002: exited on signal 6
pid 21731 (imap), uid 1002: exited on signal 6

Sometimes the dovecot process serving an user crashes. He runs mostly
airmail on macOS and samsung mail on Android.

Happens only to that particular user.

The log shows the following: (maillog)

Dec 16 14:52:35 mail dovecot: imap-login: Login: user=<USER>, method=PLAIN,
rip=79.12.201.157, lip=192.168.1.28, mpid=74779, TLS,
session=<wMQI+iN9VtRPDMmd>
Dec 16 14:52:39 mail dovecot: imap(USER)<74441><nLa+7SN9+fhPDMmd>: Panic:
file charset-iconv.c: line 83 (charset_to_utf8_try): assertion failed:
(srcleft <= CHARSET_MAX_PENDING_BUF_SIZE)
Dec 16 14:52:39 mail dovecot: imap(USER)<74441><nLa+7SN9+fhPDMmd>: Fatal:
master: service(imap): child 74441 killed with signal 6 (core not dumped -
https://dovecot.org/bugreport.html#coredumps - set service imap {
drop_priv_before_exec=yes })

I'm here if more info is needed.

Thanks for taking a look.
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