You need to remove the password in the Sharing tab in the folder settings. It is only needed if you intend to share the folder and accept it on the other side as “Receive Encrypted”. See https://github.com/syncthing/syncthing/issues/9784 for details.
Unfortunately, one side of the sync is a rented cloud server on whatbox. I don’t have the choice to remove the password. When I do, it says in the password dialog : Device is untrusted, enter encryption password
And I cannot save until I enter a password. The whatbox server is running 1.27.12, and I don’t have the option to upgrade it that I see.