I would recommend always building an All-In-One deployment in a virtual machine so that you have a reference to compare against when moving away from the 'stock config'. Documentation for the AIO can be found here https://docs.openstack.org/openstack-ansible/ussuri/user/aio/quickstart.html

I gradually decreased the complexity of my test system towards the 'stock config' to find out what caused the error but it kept showing up. Finally, it vanished when I rebooted the server and my original configuration also completed successfully afterwards. 🤦‍♂️ Thank you for your help and the insights to OSA! Kind regards, Oliver