Sieve can't move messages in a public namespace

Hello,

my dovecot version is 2.1.17

I've configured some public namespaces (config attached). When a mail arrives at a certain mail address, that mail is handled by dovecot lmtp server and a sieve script is executed that shuold move the message in one of the namespaces. This is the script:

require "fileinto";

if header :contains "From" "example@example.com" { fileinto :create "Cond-UFF_FORNITORI/CONTABILITA/Inviata"; } else { fileinto :create "Cond-UFF_FORNITORI/CONTABILITA/Ricevuta"; }

The problem is that the sieve script fails with the error:

sieve: info: started log at Jun 27 12:42:28.
error: msgid=<c9154db4-ff68-52a3-2639-11f09f225f68@fcr.re.it>: failed to store into mailbox 'Cond-UFF_FORNITORI/CONTABILITA/Ricevuta': Mailbox doesn't exist: Cond-UFF_FORNITORI/CONTABILITA/Ricevuta.

I double checked config, dovecot documentation, sieve documentation, ACL, filesystem paths, filesystem permissions, etc. etc. I'm pretty sure it's all right. So why isn't sieve working?

Thanks if you bother to answer & Cheers

Paolo