Steam Validation Rejected?

[kakoka]

So yeah. I launch up the game, and select continue game so I can play on my last save. But it gives me this: "Steam Validation Rejected" I did not pirate the game. It was gifted to me. This didn't happen before. It doesn't happen on my other PC. Any ideas why this happens? By the way, if this is important: I have a hard drive, 2 partitions. Steam and the games are installed in the second, D: partition. Running Windows 7 64 bit.

[Hunter #357]

You must not have validated your Steam correctly.

[cheshirm]

your last save - were you playing co-op with someone and now arent?