[Dovecot] "[CANNOT] Mailbox GUIDs are not permanent without index files" when trying to delete folder

[Nissim Etrog]

Hello,

Dovecot 2.0.9  on   Red Hat Enterprise Linux Server release 6.4 (Santiago)

location is set to :
mail_location = mbox:~/mail:INBOX=/var/mail/%u:INDEX=MEMORY

Index is set to memory due to permissions problems when using disk index.

Everything works, even rename folder, but when trying to delete folder, using any mail client, it fails with:
"[CANNOT] Mailbox GUIDs are not permanent without index files".

Why ?

Regards,Nissim Etrog