The BIM Server Manager will be Offline and cannot be restarted with the configurator if the computer which is hosting the BIM Server 20 build no. 3012 is upgraded to macOS 10.12 Sierra.
Solution before upgrading to Sierra:
Install the latest ARCHICAD 20 Update 4012 (not yet tested for all language versions) before Upgrading to macOS 10.12 Sierra. Before upgrading to Sierra, create backup from the BIM Server. For further information, see the BIM Server Backup Guide
Workaround after upgrading to Sierra:
If the Operating System is already upgraded to Sierra while the BIM Server's build number was 3012 and the Server is not functional then the computer have to be rolled back to the previous Operating System and restore the BIM Server from a backup.