This I found on the game4game website:
http://www.game4game.at/shop/catalog...5f3f9ae23572f9
This is the important part(posted 02.09.2011):
"Nur bei uns erhältlich: Die Limited Edition und die Limited
Treasure Edition. ... Beide
Editionen wurden mit freundlicher Unterstützung von Deep Silver realisiert, eine
offizielle Lizenz gibt es allerdings auf keiner Sonderedition von Dead Island."
Trying to translate it:" Only available here: The Limited Edition and the Limited Treasure Edition. .... Both editions were achieved with friendly support by Deep Silver. There is no official license on any Special edition of Dead Island."
I asked on Facebook what has happend to the certificate. They replied this:"Das Zertifikat musste leider gestrichen werden, sorry dafür."
Translation:"The certificate unfortunately had to be cancelled, sorry for that!"