Hey @AlexHofbeck,
I checked my nginx configuration against your config snippet and found that I missed the proxy header forwarding for the backend location (here speckle-server/utils/docker-compose-ingress/nginx/default.conf at 99bf268a7470bbf2e405251e379c8e87f38e515a · specklesystems/speckle-server · GitHub). After adding these lines to the backend location config, the auth finally worked successfully.
The current configuration is only intented for a test stage. For production I like to switch to k8s.