I also had lots of trouble logging into the forums just now. It took me six tries. I wonder if it's related.
Am I supposed to be able to log in to the "Sign in with your PlayFab ID" part with logins of this forum?
You should use the same login in the forums and the game. However based on the fact that I had to unlock your post due to your very low post count (spam protection) the account "vatas" does not own the game. You should have another account on which you own the game?
I don't have the game on steam like that, I can only guess it should have you create an account somehow. No idea how that works.
If you ever played PA in past then you should have PlayFab account already. If no then game should let you register PlayFab account on first launch, but rarely this not working. In this case you can register PlayFab account here: https://uberent.com/User/RegisterDialog Link it to Steam here: https://uberent.com/user/LinkSteam