@DaRon Thank you for the two hints!
Concerning the first one, what ALLOW_ADMIN_EMAIL_LOGIN=y does is to give immediate access to other users’ mailboxes. It is not what I meant.
I thought the admin account has a mailbox too. And I wanted to get access to it via SoGo through the upper-right menu App->Webmail. But this menu item redirects back to the admin dashboard.
Thus, is it right that the admin doesn’t have any mailbox at all? It isn’t listed under the Mailboxes tab anyway.
Thank you, José