[Dovecot] dsync Error: Mailbox INBOX: Save commit failed: Mailbox was deleted under us

[Alan McGinlay - SICS]

Hi,

I am doing a mass migration of users from Cyrus imap on a solaris 
server to dovecot 1:2.2.9-1ubuntu1 and am getting this weird issue with 
dsync if I try to do a "full" sync. Debug output below:

dsync(user at example.com): Error: Mailbox INBOX: Save commit failed: 
Mailbox was deleted under us
dsync(user at example.com): Debug: brain M: out box 'INBOX' 
recv_state=recv_last_common send_state=done changed=1
dsync(user at example.com): Debug: brain M: out state=sync_mails changed=1
dsync(user at example.com): Debug: brain S: in state=sync_mails
dsync(user at example.com): Debug: brain S: in box 'INBOX' 
recv_state=recv_last_common send_state=done
dsync(user at example.com): Debug: brain S: out box 'INBOX' 
recv_state=recv_last_common send_state=done changed=0
dsync(user at example.com): Debug: brain S: out state=sync_mails changed=0
dsync(user at example.com): Debug: imapc(10.10.10.10:143): Disconnected
[2]+  Killed                  /usr/bin/dsync -v -o mail_fsync=never 
backup -R -f -u user at example.com imapc:

(I changed the domain / ip here of course)

I notice that it does the IMAP sync for a while and then shows 
connecting info for pop3 (I have configuration for both imap and pop). 
The user in question has never connected with POP.

imap migration config:

imapc_host = 10.10.10.10
imapc_user = %u
imapc_master_user = cyrusadmin
imapc_password = blahblahblahpasswordhere
imapc_features = rfc822.size
imapc_features = $imapc_features #fetch-headers
mail_prefetch_count = 20

pop configuration:

pop3c_host = pop.example.com
pop3c_user = %u
pop3c_master_user = cyrusadmin
pop3c_password = blahblahblahpasswordhere

namespace {
   prefix = POP3-MIGRATION-NS/
   location = pop3c:
   list = no
   hidden = yes
}
protocol doveadm {
   mail_plugins = $mail_plugins pop3_migration
}
plugin {
   pop3_migration_mailbox = POP3-MIGRATION-NS/INBOX
}

Thanks in advance for any pointers on this!

/Alan