Apparently you already have your account configured. Why would you commence on a reinstall of that account? So lets forget all the information about the previous topic you found as it mostly does not apply to you as you a;ready have an account in Thunderbird. I do however think you need to contact the proton folks for instructions on how to use their software. 127.0.0.1, also known as "localhost," is a special loopback IP address used by a computer to refer to itself. Network traffic sent to this address is not sent out to the physical network but is instead looped back internally by the operating system. My guess is someone is using encrypted connections on the local device (Proton) and you simply have no legitimate encryption certificate for your computer because you simply do not need one until someone decides you need to use encrypted connections. You might want to ensure that the bridge is not using SSL, that broken encryption protocol has not been used in Thunderbird for some 10 years. https://proton.me/support/comprehensive-guide-to-bridge-settings you might also need to manually import the encryption certificate as mentioned in that linked document.