Dovecot panic for one user after upgrading

[Owen B. Mehegan]

The EC2 instance that I run Dovecot on (personal mail server) was 
forcibly "retired" yesterday, and as a result I ended up rebuilding from 
backups. In the process I switched from Ubuntu Precise to Ubuntu Xenial 
(12.04 to 16.04), and this in turn caused me to upgrade Dovecot from 
2.0.19 to 2.0.22. Now, although accessing mail seems to work OK using my 
various clients, I am seeing these errors in my logs:

Jan  3 05:56:58 -- dovecot: imap(owen): Panic: file 
mail-index-sync-keywords.c: line 227 (keywords_update_records): 
assertion failed: (data_offset >= sizeof(struct mail_index_record))
Jan  3 05:56:58 -- dovecot: imap(owen): Error: Raw backtrace: 
/usr/lib/dovecot/libdovecot.so.0(+0x7aee2) [0x7fe9c08dbee2] -> 
/usr/lib/dovecot/libdovecot.so.0(+0x7afcd) [0x7fe9c08dbfcd] -> 
/usr/lib/dovecot/libdovecot.so.0(i_fatal+0) [0x7fe9c0880a61] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(mail_index_sync_keywords+0x828) 
[0x7fe9c0c1fdd8] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(mail_index_sync_record+0xef) 
[0x7fe9c0c2079f] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(mail_index_sync_map+0x238) 
[0x7fe9c0c21758] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(mail_index_map+0x71) 
[0x7fe9c0c18de1] -> /usr/lib/dovecot/libdovecot-storage.so.0(+0xd28fd) 
[0x7fe9c0c2e8fd] -> /usr/lib/dovecot/libdovecot-storage.so.0(+0xd2f03) 
[0x7fe9c0c2ef03] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(mail_index_open+0x78) 
[0x7fe9c0c2efe8] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(index_storage_mailbox_open+0x92) 
[0x7fe9c0c08042] -> /usr/lib/dovecot/libdovecot-storage.so.0(+0x5e8d2) 
[0x7fe9c0bba8d2] -> /usr/lib/dovecot/libdovecot-storage.so.0(+0x5e9b8) 
[0x7fe9c0bba9b8] -> /usr/lib/dovecot/libdovecot-storage.so.0(+0x37b86) 
[0x7fe9c0b93b86] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(mailbox_open+0x20) 
[0x7fe9c0b93d00] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(index_storage_get_status+0x20) 
[0x7fe9c0c073b0] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(mailbox_get_status+0x37) 
[0x7fe9c0b940e7] -> dovecot/imap(imap_status_get+0x77) [0x55d3b4c3f977] 
-> dovecot/imap(cmd_status+0x1a9) [0x55d3b4c32969] -> 
dovecot/imap(command_exec+0x9c) [0x55d3b4c3806c] -> 
dovecot/imap(+0x193e2) [0x55d3b4c363e2] -> dovecot/imap(+0x1946c) 
[0x55d3b4c3646c] -> dovecot/imap(client_handle_input+0x165) 
[0x55d3b4c36825] -> dovecot/imap(client_input+0x82) [0x55d3b4c36db2] -> 
/usr/lib/dovecot/libdovecot.so.0(io_loop_call_io+0x4c) [0x7fe9c08efc6c] 
-> /usr/lib/dovecot/libdovecot.so.0(io_loop_handler_run_internal+0x101) 
[0x7fe9c08f1101] -> 
/usr/lib/dovecot/libdovecot.so.0(io_loop_handler_run+0x25) 
[0x7fe9c08efcf5] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_run+0x38) 
[0x7fe9c08efe98]
Jan  3 05:56:59 -- dovecot: imap(owen): Fatal: master: service(imap): 
child 18289 killed with signal 6 (core dumped)

I have deleted the dovecot.* files in the Maildir for all users (did 
this while Dovecot was stopped, then started it again), but the errors 
persist. They are especially frequent for one particular user. I can't 
figure out where the core file is, it doesn't seem to be in my home or 
in /var/run/dovecot

Here's my dovecot -n output:

# 2.2.22 (fe789d2): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.13 (7b14904)
# OS: Linux 4.4.0-1043-aws x86_64 Ubuntu 16.04.3 LTS
mail_location = maildir:~/Maildir
namespace inbox {
   inbox = yes
   location =
   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 {
   driver = pam
}
protocols = " imap"
service auth {
   unix_listener /var/spool/postfix/private/auth {
     group = postfix
     mode = 0660
     user = postfix
   }
}
ssl_cert = </etc/letsencrypt/live/mydomain/cert.pem
ssl_key = </etc/letsencrypt/live/mydomain/privkey.pem
userdb {
   driver = passwd
}

Thanks in advance for any support you can offer!