I have a similar problem. After an upgrade from v18 (Windows) to v20 (Linux), I do not see the admin interface, when logging in with the System Owner account.
What I did so far:
Before upgrading, I assigned the "system owner" to an extension and gave it all permissions to access the admin interface. After this, I resetted the Extension Password and I maked shure, that the login (to v18) with System Owners extension was possible. Then I exported the config and imported it to V20 on Linux.
After the upgrade, I can login with my System Owner but the "Admin"-option is still missing. What I am doing wrong?
Best regards, Christian