dovecot - Jul 2017 - failed to store into mailbox 'INBOX/Junk': Permission denied

> For more clarity I also attach the content of
> "/etc/dovecot/conf.d/15-mailboxes.conf"
>
>
> #  mailbox Junk {
> #    special_use = \Junk
> #  }
> #  mailbox Trash {
> #    special_use = \Trash
> #  }
>   mailbox INBOX/Junk {
>   auto = subscribe
>   special_use = \Junk
>   }
>   mailbox INBOX/Trash {
>   auto = subscribe
>   special_use = \Trash
>   }
>
>   Date: Mon, 24 Jul 2017 18:06:29 +0200
>   From: Christian Kivalo <ml+dovecot at valo.at>
>
>   Please also provide your doveconf -n
Here my Dovecot configuration (I've commented out "mailbox Junk"
and
"mailbox Trash" above)

# 2.2.13: /etc/dovecot/dovecot.conf
# OS: Linux 3.16.0-4-amd64 x86_64 Debian 8.5 ext4
auth_mechanisms = plain login cram-md5
mail_location = maildir:/var/vmail/%d/%n/Maildir
mail_plugins = acl
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 ihave
namespace inbox {
   inbox = yes
   location    mailbox Drafts {
     special_use = \Drafts
   }
   mailbox INBOX/Junk {
     auto = subscribe
     special_use = \Junk
   }
   mailbox INBOX/Trash {
     auto = subscribe
     special_use = \Trash
   }
   mailbox Junk {
     special_use = \Junk
   }
   mailbox Sent {
     special_use = \Sent
   }
   mailbox "Sent Messages" {
     special_use = \Sent
   }
   mailbox Trash {
     special_use = \Trash
   }
   prefix }
passdb {
   args = /etc/dovecot/dovecot-sql.conf.ext
   driver = sql
}
plugin {
   acl = vfile:/etc/dovecot/dovecot-acl
   sieve = ~/.dovecot.sieve
   sieve_after = /etc/dovecot/sieve-after
   sieve_dir = ~/sieve
}
protocols = " imap lmtp sieve pop3"
service auth {
   unix_listener /var/spool/postfix/private/auth {
     group = postfix
     mode = 0660
     user = postfix
   }
}
service lmtp {
   unix_listener /var/spool/postfix/private/dovecot-lmtp {
     group = postfix
     mode = 0600
     user = postfix
   }
}
ssl_cert = </etc/letsencrypt/live/server.sio4.org/fullchain.pem
ssl_cipher_list = 
ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA
ssl_dh_parameters_length = 2048
ssl_key = </etc/letsencrypt/live/server.sio4.org/privkey.pem
ssl_prefer_server_ciphers = yes
userdb {
   args = uid=vmail gid=vmail home=/var/vmail/%d/%n
   driver = static
}
protocol imap {
   mail_plugins = acl imap_acl mail_log notify
}
protocol lmtp {
   mail_plugins = acl sieve
}

Many thanks Christian!