Traefik on Kubernetes does not keep path over redirect
The issue here is that PathPrefixStrip removes /config-server and it initially forwards the request to your backend with just /. However, when your backend redirects it's a new request and the Ingress doesn't understand the request to / or /login. You could create two Ingress(es) for each backend and use single backend in each with a path to /. But this would mean adding a new ingress-class and the good news is that Traefik supports it. You could also try an nginx ingress which has the --ingress-class option.
You can find more information on how to create multiple ingress controllers here
If you want a single Ingress it will also be tricky since you might have to add logic in your app to understand the /config-server path for example.