I suspect the problem may be that during the initial setup when asked if I trusted the certificate for 127.0.0.1:1143 I checked “don’t ask again” and inadvertently clicked “Don’t Connect.
Yes agree that does sound the problem as yes you normally need to press the Install Certificate usually twice to get the Proton bridge to connect to incoming mail as per @Gary thread post below.
So as you have already tried deleting and re-adding the account and the certificate now just doesn’t appear again, I would then suggest (if this is just a brand new install of eM Client) to uninstall eM Client and then “delete the eM Client database hidden folder completely” and then start again from scratch with a new clean install.
Hidden database folder in Windows is “Users\yourusername\AppData\Roaming\eM Client”
Hidden database folder in Mac is “Users\yourusername\Library\Application Support\eM Client”
(Proton Bridge Mail Setup