That's strange. I went to your page www.b2bcontact.nl then clicked 'Inloggen' and clicked on the LinkedIn button underneath the cover picture in JomSocial. I was able to create an account, log out, and then log in afterwards just fine. Username is sourcecoast and you should be able to see that it is mapped to my LinkedIn account in JFBConnect > User Map.
What exact URL are you on when you're attempting to log in? Any other details that you can provide to reproduce this?