On 30 Sep 2019, at 14.07, tovises via dovecot <dovecot@dovecot.org> wrote:
Tath's help! But something still missing :(

$ sudo doveadm -D -o imapc_user=tovis -o imapc_password=<password> -o
imapc_host=nusi -R -u tovis imapc:

Exited with messages (excerption - too many private information exposed):

dsync(tovis): Debug: brain M: Mailbox INBOX.threatposts:
local=00000000000000000000000000000000/0/0,
remote=a557d4d9d858c0ceceef5cd10a973bdc/0/1: mailbox not selectable yet
dsync(tovis): Debug: brain M: Mailbox INBOX.todo:
local=00000000000000000000000000000000/0/0,
remote=891e2fe9dc8874c0a19e496acf802566/0/1: mailbox not selectable yet

Remote (courier) refuses A SELECT INBOX.threatpost and A SELECT INBOX.todo

dsync(tovis): Debug: brain M: Deleting mailbox 'INBOX' (GUID
2e829f01d5cc915dd103000056dcb805): UIDVALIDITY changed (1454614013 ->
1569836245)
dsync(tovis): Error: Mailbox INBOX sync: mailbox_delete failed: INBOX
can't be deleted.

IMAP RFC mandates that if UIDVALIDITY changes then the whole mailbox needs to be resynced.
Dovecot then tries to delete INBOX for resync, but INBOX deletion is not allowed.

The only way to recover is to clear out the target completely and restart from beginning.

btw, 1454614013 = Thu Feb  4 20:26:53 CET 2016
and  1569836245 = Mon Sep 30 11:37:25 CEST 2019

so did you actually create INBOX on target system before the initial sync?

Sami