Edit:
I'm happy to announce that the Steam servers have been fixed. You can now play Dark Hours with others by creating and joining lobbies as usual!
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Steam servers, specifically Steam connection managers were down today, and the Steam network services have still issues preventing Steam players to play in multiplayer to different games, Dark Hours included.
Please be patient the time Steam solves their issues, we are not the only game impacted by this problem. This is global and on the Steam end, only them can fix it.
You probably won't be able to join lobbies till they have not fixed their servers as Dark Hours is using the Steam connections to make people play together.
Again only Valve / Steam knows when it'll be fixed as this is on their backend / servers. As connection managers are back online, we hope that Steam multiplayer relay servers will get fixed soon...
Sadly, we have no official information how long it will impact Dark Hours players
We have contacted the Steam support team and waiting for their answers
I'm sorry for the inconvenience, I'll keep you updated,
Raven