[Samba] problems setting a public share

[Luca Ferrari]

Hi all,
my samba server has security set to user, and it works for each share I 
specify a valid users list. However, the following public share does not 
work:

[PUB_SMB]
        comment           = Cartella pubblica (Mac e PC) - Non viene fatto il 
backup
        path              = /mnt/samba/pub_smb
        browsable         = yes
        available         = yes
        writable          = yes
        printable         = no
        force group       = pub
        guest ok          = yes
        public            = yes

I found in the logs that 
make_connection: connection to PUB_SMB denied due to security descriptor.

If I connect to the share authenticating as a user I can log-in, but as a 
guest no. I've tried to delete the file /var/lib/samba/share_info.tbd, but 
once I try to reconnect the file appears again, and even when the file is 
absent I cannot connect to the share.

Any idea?

Thanks,
Luca