I think that there is a bug with the forums: if one is logged on with a Facebook account, it is not possible to post messages onto the forum. In the "settings" part of the forum, one gets the following message:
xxx you do not have permission to access this page. This could be due to one of several reasons: "Your user account may not have sufficient privileges to access this page.
Are you trying to edit someone else's post, access administrative features or some other privileged system?
If you are trying to post, the administrator may have disabled your account, or it may be awaiting activation.
Indeed I never received an activation message (since I am logged with Facebook), so that's probably what causes this bug. For the time being, I have open a new account with an email address to post here.
Note that I've already reported this issue by email to the customer service, and they answered me one week after to contact them by phone...