I have little time at the moment. But have you installed the proton bridge software? Used the troubleshooting information supplied by proton? https://proton.me/support/mail I would guess that on of the most common issues that arises with Proton after installing the bridge is the use of self signed SSL/TLS certificates in the product. Try changing the connection security to none. I make this suggestion of none in the full knowledge that the encryption only related to a connection between two software products on your computer. Personally I fail to understand the purpose of encryption in this instance unless it is Protons opportunity to claim end to end encryption. If your device is compromised, then the certificates will be available to those that compromised the system so it serves no real security purpose. Unlike communication between the bridge software and the proton servers. But you settings in Thunderbird have nothing to do with that.