By the error I take this is not using imapsync but a migration tool provided by Office 365 that tries to connect to NethServer IMAP server with wrong user credentials or with a user with insufficient permissions.
Same error as in these cases: Migration to MS 365 error IMAP / Dovecot and Migrating to office 365 error Dovecot
Do not know the solution but you can check nethserver mail logs (and the ones the migration tool might provide) to get more info about how the migration tool is trying to connect and why it was denied access when trying to SELECT an IMAP mailbox/folder.