Could you provide detailed steps on how you're trying to log in via LinkedIn? I tried on your site with the following and was able to create a user and log in properly:
1. Go to
www.openupgroup.nl/cms
2. Click Aanmelden menu link.
3. Click on the LinkedIn button on the page
4. Approve permissions on LinkedIn.
5. That brought me back to JFBConnect's login / register page.
6. Set a username
7. Hit the register button.
8. User was created and logged in properly (visited the Joomla Users component - profile page to see my details)
-Melissa