d'après le lien du forum steam consacré à ce problème, les admins travaillent dessus et devraient apporté la solution dans 1 ou 2 heures.
"Hello everyone!
Most of you are probably experiencing this message "Not subscribed (do not own the game). Buy!?"
http://i.imgur.com/RgptYKY.png
We are working on a fix. Expect it to take between 1-2 hours.
Sorry for any inconvenience and thanks for being patient!
Almir
OVERKILL - a Starbreeze Studio."