[Zope] privileges problem

[Catonano]

Hello all,

I'm tring to execute this script:

lista = []
folder = container.indice2() #indice2 is a ZCatalog
for file in folder:
 if not file.hasProperty('codice'):
  lista.append(file.id)
  attributes=getattr(folder, file)
  attributes.manage_addProperty('codice', 'inserted yet', 'string') 
return lista

I borrowed the idea from the /Examples/GuestBook/addEntry  script

Zope prompts me for a user name and passwd, I type my usual account 
data (I'm the only user and I think I have full privileges) and BAM!

Zope Error

Zope has encountered an error while publishing this resource.

Error Type: Unauthorized
Error Value: You are not allowed to access in this context

Troubleshooting Suggestions

    * The URL may be incorrect.
    * The parameters passed to this resource may be incorrect.
    * A resource that this resource relies on may be encountering an 
error.

For more detailed information about the error, please refer to the HTML 
source for this page.

If the error persists please contact the site maintainer. Thank you for 
your patience.

I tried to give the script all the proxy roles I could
I opened Control Panel/acl_users and gave manage property permission to 
managers and owners (I'm a manager user): NOTHING!

Please: what's going on?

Thanks anybody who will help me
Bye
Catonano