Mantis requires you to register an account on it. The login screen with the Introversion logo requires your login information that you use here, the second login screen is where you create the new account for Mantis. For convenience, just use the same forum name and password. The bug bash video tha...

It seems like I can't login with my forum credentials. I always get the error "Failed to authenticate user. Did you type in your username / password correctly?" that is also shown when I just type in random words. My password only consists of numbers and letters (capital and non-capital), ...

I only own the game since alpha 13 was released, so i can't say what was the situation before it, but i have the same issue. My os is Win7 x64 and i have no problems accessing the workshop itself (neither in the Steam client nor ingame).