Updated core installation - now recycling "onboarding"

I naively upgraded my HA core installation on my Mac Mini to the latest version 2023.9.1 and now the web site just reloads an onboarding URL instead of Lovelace. I tried downgrading to 2023.6.1 but the problem persists.

a) HALP!

b) Tried going VirtualBox instead, but how do I get my old configuration in there - I have no formal HA-backup, can I create a backup manually from files in some manner? Also, how do I ssh into the VirtualBox installation - there is no Supervisor in the .vdi-installation linked at MacOS - Home Assistant . I need to install custom integrations, including a manual hack to get two SolarEdge Modbus installations.

I restored my compuer backups of the install and the older version turned out to be 2023.4.1.

Apparently something broke between 4.1 and 9.1. Too big an upgrade in a single leap?