hi @Phil_DI welcome to the forum 
please take a look at this guide and work through the checklist
Nextcloud Collabora integration
The error
looks like you are trying to access some non-existing host/path/URL…
btw - if you are running systems behind reverse proxy you don’t need to expose container ports to the network:
using expose: ports connect only to the docker network and don’t connect with the host network making the system little more secure.