The gameheaven url is just showing as a link when you post in on a forum page. That has nothing to do with how it is addressed or handled in a cookie file on you disk.
shall I be the guy who helps you out of all this mess ?

The problem is with the domain / site name of the cookie. When you both go to gameheaven you probably type in the DNS name and not the www name. resulting in you going to the URL
http://gameheaven.nl. And not
http://www.gameheaven.nl. The cookie is set for
www.gameheaven.nl. And guess what. If you click on a link for one of the boards [ if you went to
http://gameheaven.nl where you cookie doesn't work ] you will be guided to
www.gameheaven.nl. Note the www in from of it while you never ever typed it in in the first place ?
So in a way the cookie is working to good. The main page and main forum page can be addressed with gameheaven.nl. Where the forum's cookie is a varification for a site called
www.gameheven.nl. Long stsory but technically those 2 sitenames don't have to correspond with the samen site. So you are not validated untill you go to a
www.gameheaven.nl link. Which will automatically happen when you go to one of the boards. Or when you have bookmarked the page as a
www.gameheaven.nl one like me. .

O, btw. . . do you both know it's possible with the cookie ID for any one to log in to this forum with your ID ?
Just modified your post so the important user info isn't in it anymore.
btw. I am currently logged in and editing one of my posts. And in another tab I just accessed the
http://gameheaven.nl/forum/ site . . not validating me as myself. . . going to a forum board I emidiately was validated. . .