Deleting not actually deleting

[abidibo]

The delete function removes e-mail from interface, but actually doesn't delete from server.
I've seen previous similar issues, but I don't think in my case it is related to aliases.

log:

D, [2016-10-11T16:27:31.972129 #1626] DEBUG -- : Flag [15524] Deleted +FLAGS
D, [2016-10-11T16:27:31.972383 #1626] DEBUG -- : Deleting uid_set: [15524]
D, [2016-10-11T16:27:31.972508 #1626] DEBUG -- : Decremented max seqno from 2963 to 2962
D, [2016-10-11T16:27:31.973080 #1626] DEBUG -- : imap.uid_copy [15524] to 
D, [2016-10-11T16:27:32.158916 #1626] DEBUG -- : Error:  No folder NIL (Failure)`

So it seems the Trash alias is not working in some way...
My configuration:

mailbox_aliases:
  all: Tutti i messaggi
  drafts: Bozze
  sent: Posta inviata
  starred: Speciali
  trash: Cestino
  important: Importanti

I'm quite sure this aliases are correct, since I can view messages inside Trash for example. The problem seems to be inside flagging_and_moving.rb, since if I log before line 22:

log "CUSTOM LOG #{ mailbox_aliases }"

the output in the log file is:

CUSTOM LOG {}

an empty dictionary? (I'm not a ruby dev). Any ideas? The mailbox_aliases is not accessible from here? Can be related to ruby version? This is mine:

abidibo@abidibo-K56CM:~$ ruby -v
ruby 2.1.8p440 (2015-12-16 revision 53160) [x86_64-linux]

Thanks for your help.