dovecot - Dec 2016 - Error while migrating mailbox with 'doveadm sync'

Dear all,

I successfully migrated hundreds of mailboxes with Dovecot 2.2.24 on OpenBSD
6.0 (amd64. Old server is OpenBSD 5.1, i386, Dovecot-2.0.17), but got error
while
migrating 3 mailboxes:

# doveadm -o mail_fsync=never sync -1 -R -s '' -u user at domain.com
imapc:
dsync(user at domain.com): Panic: file mailbox-alias-plugin.c: line 77
(mailbox_symlink_exists): assertion failed: (ret > 0)
Abort trap

Same error while running ?doveadm ? backup ?'.
What does this error actually mean? How can i fix it?

- Here?s full log running doveadm with ?-D? option: http://pastebin.com/Um5fbuNM
- ?dovecot -n? on old server: http://pastebin.com/4E6xzFKU
- ?dovecot -n? on new server: http://pastebin.com/4uHwQnMN

I checked release notes of Dovecot 2.2.25/26/27, there?re 3 fixes related to
'doveadm sync?, but not sure whether it helps fix this issue.

2.2.26:
+ doveadm sync/backup: Added -I <max size> parameter to skip too large
mails.
+ doveadm sync/backup: Fixed -t parameter and added -e for "end date?.

2.2.27:
- doveadm sync -1: Fixed handling mailbox GUID conflicts.

> On Dec 18, 2016, at 8:26 PM, Zhang Huangbin <zhb at iredmail.org>
wrote:
> 
> # doveadm -o mail_fsync=never sync -1 -R -s '' -u user at
domain.com imapc:
> dsync(user at domain.com): Panic: file mailbox-alias-plugin.c: line 77
(mailbox_symlink_exists): assertion failed: (ret > 0)
> Abort trap
Checked IMAP folders on old server, this user has ?.Sent? and ?.Sent Messages?
on old server, but ?.Sent Message? is configured to be a alias mailbox on new
server. After merge 2 folders, then sync works fine.

Sorry about the noise.