doveadm backup doesn't transfer mail from root INBOX

I'm trying to migrate IMAP mails from dovecot 1.1.20apple0.5 (osx) to dovecot 2.2.33.2 (ubuntu). I'm using "doveadm backup" to migrate my data. It works fine for all subfolders, but the root INBOX stay empty on the new server. I suspect a problem related with hierarchy separator ("." on previous server, "/" on new) or with the namespace prefix (set to "INBOX/" on new server). Any hint?

show there is no messages in INBOX:

# doveadm mailbox status -u (myuser) all INBOX
INBOX messages=0 recent=0 uidnext=1 uidvalidity=1553630060 unseen=0 highestmodseq=1 vsize=0 guid=d54cd42849739b5cb5790000ed87e144 firstsaved=18446744073709551615

show there is messages in subdirectory:

# doveadm mailbox status -u (myuser) all INBOX/Sent
INBOX/Sent messages=253 recent=0 uidnext=8721 uidvalidity=1263328366 unseen=0 highestmodseq=256 vsize=16335358 guid=bfb2e03fdce327671e82bf173b1ccb8b firstsaved=1553634631

The command I'm runnning on the new server to migrate data:

# doveadm -o mail_fsync=never backup -R -u (myuser) imapc:

My config on the new server:

# doveadm -n
# 2.2.33.2 (d6601f4ec): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.21 (92477967)
# OS: Linux 4.15.0-46-generic x86_64 Ubuntu 18.04.2 LTS 
auth_default_realm = example.com
doveadm_password =  # hidden, use -P to show it
doveadm_port = 53683
first_valid_uid = 200
imapc_features = rfc822.size fetch-headers
imapc_host = oldmail.example.com
imapc_master_user = (removed)
imapc_password =  # hidden, use -P to show it
imapc_user = %n
last_valid_uid = 200
mail_location = mbox:~/mail:INBOX=/var/mail/%u
mail_plugins = " notify replication zlib quota"
mail_prefetch_count = 20
mail_privileged_group = mail
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character vacation subaddress comparator-i;ascii-numeric relational regex imap4flags copy include variables body enotify environment mailbox date index ihave duplicate mime foreverypart extracttext
mdbox_rotate_size = 10 M
namespace inbox {
  inbox = yes
  location = 
  mailbox "Éléments envoyés" {
    special_use = \Sent
  }
  mailbox "Éléments supprimés" {
    special_use = \Trash
  }
  mailbox Brouillon {
    special_use = \Drafts
  }
  mailbox "Courrier indésirable" {
    special_use = \Junk
  }
  mailbox Drafts {
    auto = subscribe
    special_use = \Drafts
  }
  mailbox Junk {
    auto = subscribe
    special_use = \Junk
  }
  mailbox Sent {
    auto = subscribe
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    auto = subscribe
    special_use = \Trash
  }
  prefix = INBOX/
  separator = /
}
passdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
plugin {
  mail_replica = tcps:mail2.example.com:53683
  quota = count:User quota
  quota_grace = 10%%
  quota_rule = *:storage=10G
  quota_rule2 = INBOX/Trash:storage=+100M
  quota_vsizes = yes
  quota_warning = storage=100%% quota-warning 100 %u
  quota_warning2 = storage=95%% quota-warning 95 %u
  quota_warning3 = storage=90%% quota-warning 90 %u
  quota_warning4 = storage=85%% quota-warning 85 %u
  quota_warning5 = storage=75%% quota-warning 75 %u
  quota_warning6 = -storage=80%% quota-warning '-80' %u
  quota_warning7 = -storage=100%% quota-warning '-100' %u
  sieve = /var/vmail/domains/%Ld/%Ln/.dovecot.sieve
  sieve_before = /var/vmail/sieve/before.sieve
  sieve_default = /var/vmail/sieve/default.sieve
  sieve_global = /var/vmail/sieve/global
  zlib_save = lz4
}
protocols = " imap lmtp sieve"
service aggregator {
  fifo_listener replication-notify-fifo {
    user = vmail
  }
  unix_listener replication-notify {
    user = vmail
  }
}
service auth {
  inet_listener {
    address = (removed)
    port = 55123
  }
  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
  }
}
service doveadm {
  inet_listener {
    port = 53683
    ssl = yes
  }
}
service imap-login {
  inet_listener imap {
    port = 0
  }
  process_min_avail = 10
}
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0660
    user = postfix
  }
}
service quota-warning {
  executable = script /usr/local/bin/quota-warning.sh
  unix_listener quota-warning {
    user = vmail
  }
}
service replicator {
  process_min_avail = 1
  unix_listener replicator-doveadm {
    mode = 0600
    user = vmail
  }
}
ssl = required
ssl_cert = </etc/ssl/example/example.com.crtbundle
ssl_client_ca_dir = /etc/ssl/certs
ssl_key =  # hidden, use -P to show it
userdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
verbose_proctitle = yes
protocol lmtp {
  mail_plugins = " notify replication zlib quota sieve"
  postmaster_address = [email protected]
}
protocol imap {
  mail_plugins = " notify replication zlib quota imap_zlib imap_quota"
}

Config LDAP

# cat /etc/dovecot/dovecot-ldap.conf.ext | grep -v "^#" | grep -v "^$"
uris = ldaps://dc1.ad.example.com ldaps://dc2.ad.example.com
dn = (removed)
dnpass = (removed)
auth_bind = yes
base = (removed)
scope = subtree
deref = never
user_filter = (&(mail=%u)(memberOf:1.2.840.113556.1.4.1941:=CN=acces_courriels,(removed))(&(|(objectclass=person))(!(UserAccountControl:1.2.840.113556.1.4.803:=2))))
pass_filter = (&(mail=%u)(memberOf:1.2.840.113556.1.4.1941:=CN=acces_courriels,(removed))(&(|(objectclass=person))(!(UserAccountControl:1.2.840.113556.1.4.803:=2))))
iterate_filter = (&(mail=*)(memberOf:1.2.840.113556.1.4.1941:=CN=acces_courriels,(removed))(&(|(objectclass=person))(!(UserAccountControl:1.2.840.113556.1.4.803:=2))))
iterate_attrs = mail=user
user_attrs = \
  =quota_rule=*:storage=%{ldap:division:10G}, \
  =home=/var/vmail/domains/%Ld/%Ln, \
  =mail=mdbox:/var/vmail/domains/%Ld/%Ln/mdbox, \
  =uid=200, \
  =gid=200