When I had this problem, it was a DNS problem. Traefik was trying to connect to the wrong IP address - an IP address that was on a network with no access from traefik.

Setting providers.docker.network solved my problem - but careful, you need to use the full network name as visible with docker network ls (in my case it was pi_traefik-web - for you it should be traefik_proxy).

In theory, you could also set the label traefik.docker.network=traefik_proxy to your nextcloud-app service.

This problem can be diagnosed through the traefik dashboard: look for the network ip a specific service is assigned -- does it show the 'public' network compatible ip?