> In November of 2019 we will stop allowing new account registrations through our ACMEv1 API endpoint. _Existing accounts will continue to function normally._
That is not true. In fact you have an account key that identifies you and the account. With that account you can create and update the certificates of many domains.
Well, that's an implementation detail for Mailinabox, which I do not have sufficient insight in but as far as I know all the domains on that machine share the same account key, and an attempt to manually update one of these gave the exact same error as the mailinabox UI. So either miab uses one account per domain (which is very well possible, but that detail is not exposed) or something else is wrong.