Ok, found the issue, it's not that I'm not allowed in some sections, it's that when I try to post a message for the first time i get this :
Only members with sufficient permission can access this page.
A security error has occurred with authentication.
Please ensure that all cookies are enabled on your web browser, you are not using a saved or cached copy of the page, and your Firewall/Proxy settings are not hiding or masking your IP address.
But if I get stubborn, go back to the previous page and try again, it works!