Tried downgrading to Geary 43.0, without success for adding the account within the application.
But by downgrading Geary further to version 40.0, the dialog to add an account actually let me configure the IMAP and SMTP servers, and I can successfully add the account. And on upgrade to the latest Geary version the account is still present.
However, it should still be possible to add said account using only the latest Geary version.
No idea how to setup geary, but protonmail bridge only supports STARTTLS (the error says that it does not support PLAIN which is one of the alternatives) with TLSv1.2 (any maybe newer) so you have to configure it. If you need the certificate for the connection it should be at ~/.config/protonmail/bridge/cert.pem or something like that.