Okay, my bad. It works perfectly. Just one thing to point out (I guess the same problem is for @mTrax): if you just set up the extension and then login using the same email address of the "First Admin" of Flarum, the system will be a bit crazy. But it's enough log out, clear cookies and login back to your website and Flarum will automatically link your account with the Admin Account in Flarum.
For users this doesn't happen - because their accounts are still not existing in Flarum, and they get created on-the-fly when logging in.
Sorry, now everything works perfectly. This extension is great. Thank you, @wuethrich.