We're using Zulip for this, which is an interesting combination of forum / chat room with strong support for threaded discussions.
You can use your piefed.social login with chat.piefed.social, BUT you need to use your email address instead of your user name. Use the same password.
The login details used by chat.piefed.social are synced with piefed.social when you log in to piefed.social or when you set a new password in your settings so if you haven't done either of those recently then log out of piefed.social and log back in.