What happened was that PC gamers discovered an exploit in the game that allowed them to get MB coins for free. For those unfamiliar, MB coins is basically the game’s in-game currency that can be bought using real-life money. This is also Konami’s way of earning extra money on top of players purchasing Metal Gear Solid V.
Safe to say the exploit would see Konami out of a lot of money had they let it continue, which is why they had decided to suspend the game until it was fixed. In a post on Steam by Robert Peeler, “Due to a possible exploit, we’ve removed access to the Beta temporarily while a hotfix is being prepared. Once the issue is addressed we’ll provide access to the Beta once again.”
No word on when exactly this will be taking place, so we guess PC gamers who were looking forward to taking the beta for a spin over the weekend might not be able to.