Actually I can tell you exactly what happened. 
You see the forum uses two addresses one is
http://forum.paradoxplaza.com/forum
The other is
http://www.europa-universalis.com/forum
So you are logged in to one of these, then you follow a link that uses the other one and you must log in again.
Just do what I do and let your computer remember both logins and you will never have a problem.
You see the forum uses two addresses one is
http://forum.paradoxplaza.com/forum
The other is
http://www.europa-universalis.com/forum
So you are logged in to one of these, then you follow a link that uses the other one and you must log in again.
Just do what I do and let your computer remember both logins and you will never have a problem.