I installed version 6.1 Build 6123 on Exchange 2019 CU12 on Server 2019
I manually added the inbound firewall rule for the admin portal (as it could not work without this)
I was blocked initially by Configuration:MFA:MFA for endpoints requiring HTTPS - even though this was enabled, and I was logged into the admin console over SSL/HTTPS with a valid certificate.
I found I had to set BOTH Admin:ConnectionSettings:https AND Admin:ConnectionSettings:Configure Access URL
The second was not obvious
Is it worth making this easier to spot?