dovecot - Nov 2021 - Error: Mailbox INBOX sync: mailbox_delete failed: INBOX can't be deleted

Hello,

I know maildir format does not allow to delete INBOX as stated in 
https://dovecot.org/list/dovecot/2016-January/102988.html. Nevertheless, 
even though I deleted the entire Maildir manually with rm -rf, doveadm 
backup fails with "Error: Mailbox INBOX sync: mailbox_delete failed: 
INBOX can't be deleted".

I would like to make a user's maildir backup to a different folder. For 
example:

rm -rf /tmp/zbbb/; doveadm backup -u test at disguised.domain.com 
maildir:/tmp/zbbb/

But I keep receiving this error message: Error: Mailbox INBOX sync: 
mailbox_delete failed: INBOX can't be deleted

Do you know what might be causing this problem?

If I use sdbox instead of maildir, the backup works:

rm -rf /tmp/zbbb/; doveadm backup -u test at disguised.domain.com 
sdbox:/tmp/zbbb/

However, I would like to use maildir format for the backup as all my 
mailboxes use this format.

Regards,