Having trouble migrating from ProtonMail using manual import config

I have Proton Bridge running, fully synced, and configured to use SSL (I've tried options here). However, no matter what I do, I get this error in FastMail's Linux app when I try to import:

Could not connect to server: IMAPTalk: socket connection failed - IO::Socket::SSL::Paranoid: Invalid internal or blocked host

I have tried 127.0.0.1 and localhost, and both fail with the same error. I've also made sure the port is fine, too.

I installed the app by downloading the Flatpak file.

If it makes a difference, I'm on Arch.