I have just installed and configured the Nextcloud version 11. Everything is working great. I have also enabled the Collabora online app and configured the same using below instructions.
Check your DNS on the server, clients and inside the container. nslookup subdomain.example.com
Check your certs on the server, clients and inside container. openssl s_client -connect subdomain.example.com:443
you can enter the container with docker exec -i -t [container-id] /bin/sh and install dnsutils there
might just be an edit to /etc/hosts and install nano to complete
docker run -t -d -p 127.0.0.1:9980:9980 -e 'domain=nextcloud\\.vote4u\\.org\\.uk' --dns=192.168.1.2 --restart always --cap-add MKNOD collabora/code
If you are on a local subnet and have local dns then you can pass that address with the above.
Its the same as the standard docker run but just passes the dns server ip with dns=192.168.1.2
docker info & docker logs will tell much and maybe post your findings as there are some distro differences.
what you can do is copy that url (the first red line) paste into a browser and you will prob find it returns the file json from nextcloud. https://… bit.
For some reason Collabora isn’t recieving the json and file and my bet is that your dns in the container is wrong.
Then maybe try and curl that from in the container and the above dns tips given.