Hello,
If you're using your Zimlet in the /_dev/ directory, the allowedDomains settings in your XML template file won't work, as that only gets set when you actually deploy your zimlet. Try doing the following from the command line:
Code:
zmprov mc <COS> +zimbraProxyAllowedDomains *
..where <COS> is a Zimbra Class Of Service identifier.
Otherwise, if you're trying to open a URL on the Zimbra server itself, it's possible that Zimbra is simply telling you that you are trying to access something that you should not be able to access with your credentials, regardless of the Zimlet proxy domains allowed.
Hope this helps.
-Rob