[Dovecot] Panic on one specific user

[LuKreme]

dovecot: imap(john at example.com): Error: /usr/local/virtual/john at example.com/dovecot.index: modseq_hdr.log_offset too large
dovecot: imap(john at example.com): Error: /usr/local/virtual/john at example.com/dovecot.index: modseq_hdr.log_offset too large
dovecot: imap(john at example.com): Panic: file mail-transaction-log-file.c: line 1148 (mail_transaction_log_file_get_highest_modseq_at): assertion failed: (offset <= file->sync_offset)
dovecot: imap(john at example.com): Fatal: master: service(imap): child 88284 killed with signal 6 (core not dumped - set service imap { drop_priv_before_exec=yes })

and also:

dovecot: lda(john at southgaylord.com): Error: /usr/local/virtual/john at southgaylord.com/dovecot.index: modseq_hdr.log_offset too large
dovecot: lda(john at southgaylord.com): Error: /usr/local/virtual/john at southgaylord.com/dovecot.index: modseq_hdr.log_offset too large
dovecot: lda(john at southgaylord.com): Panic: file mail-transaction-log-file.c: line 1148 (mail_transaction_log_file_get_highest_modseq_at): assertion failed: (offset <= file->sync_offset)

(this block does not generate a fatal error)

It doesn't appear to be affecting the user's ability to check mail (or at least no complaints about that).

The only instances of "LDA" in my logs are this error. The user is a virtual user in MySQL like all the other virtual, including other users on the domain who do not show this problem.

I think I may have posted about this before (last month?), but I haven't found a solution.

 $ dovecot -n
# 2.2.9: /usr/local/etc/dovecot/dovecot.conf
# OS: FreeBSD 9.1-RELEASE i386  
auth_mechanisms = PLAIN LOGIN
disable_plaintext_auth = no
first_valid_uid = 89
login_log_format_elements = user=<%u> %r %m %c
mail_location = maildir:~/Maildir
mail_max_userip_connections = 90
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 {
    special_use = \Drafts
  }
  mailbox Junk {
    auto = subscribe
    special_use = \Junk
  }

  mailbox NotJunk {
    auto = subscribe
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix = 
}
passdb {
  driver = pam
}
passdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
protocols = imap sieve
service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
  }
}
service imap-login {
  inet_listener imaps {
    port = 993
    ssl = yes
  }
}
ssl_cert = </etc/ssl/certs/dovecot.pem
ssl_key = </etc/ssl/private/dovecot.pem
userdb {
  driver = passwd
}
userdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  default_fields = uid=vpopmail gid=vchkpw mail_location=/usr/local/virtual/%u mail=maildir:/usr/local/virtual/%u
  driver = sql
}

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