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@domain.com imapc: dsync(user@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.