Hey there,
I found this because this seems to be my problem - I hope, one of can figure out what is wrong!?
Containers that I have running:
portainer: 172.17.0.2 8000:8000
vaultwarden: 172.17.0.5 8016:80
nginx-proxy-manager: 172.17.0.4 8081:81
nextcloud28-app: 172.17.0.6 8060:80
mailcowdockerized-nginx-mailcow-1: 172.22.1.8 8080:8080
portainer, nginx-proxy-manager and mailcowdockerized are working!
vaultwarden and nextcloud28-app do not work!
All 5 of the above containers are member of the default bridge docker network:
… so they should see each other by using their IP addresses - but some do work and some do not and I cannot understand, what is wrong!?
Here is portainer as an example, which works like a charm:
But vaultwarden and nextcloud do look like this:
I have been trying this for 16 hours now and am very desperate 
Does anyone have any helpful ideas? I would be very grateful!