After investigation, the official have confirmed several of the reasons why players receive the "Server is Full" error.

The following are known reasons why players receive "Server is Full":

-launching the game from the website
-trying to play the game without activating a digital key
-issues with the account in the database

The official are working to resolve the first and the last issue as fast as they can for all first Closed Beta users. I think the issues can be solved asap, we can enjoy the game better.