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

Hello to everyone,

On Debian Jessie (no SElinux) and "dovecot-sieve 1:2.2.13-12~deb8u1" 
I've changed the sieve (sieve_after) Junk and Trash path, from:

"INBOX.Junk"
to
"INBOX/Junk"


compiled it:

sievec /etc/dovecot/sieve-after/spam-to-folder.sieve

but now if i look into mail.err log, found this:


Jul 24 07:22:01 server dovecot: lmtp(6823, 
info at hotelsangiorgioriccione.com): Error: p/3kL/mDdVmnGgAAS3B7vQ: sieve: 
msgid=<887291195.6699.1500873334921.JavaMail.trend at backend-prod-003>: 
failed to store into mailbox 'INBOX/Junk': Permission denied
Jul 24 07:22:02 server dovecot: lmtp(6823, 
info at hotelsangiorgioriccione.com): Error: p/3kL/mDdVmnGgAAS3B7vQ: sieve: 
execution of script /etc/dovecot/sieve-after/spam-to-folder.sieve 
failed, but implicit keep was successful

and many others..


This is the "spam-to-folder.sieve content":

require ["fileinto","mailbox"];

if header :contains "X-Spam-Flag" "YES" {
  fileinto :create "INBOX/Junk";
  stop;
}


where I'm wrong?

many thanks!