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Are you running that cURL test from inside of the Nextcloud container?
Essentially, make sure that your Nextcloud container is not bypassing the reverse proxy to reach the Collabora container.
Depending on what instructions you followed, it is possible for the Collabora container to have HTTPS enabled on it as well, but it would not be using the LE certs deployed on your reverse proxy.
Edit: To test my theory: IIRC there’s also an app setting called disable_certificate_verification or the like (in the richdocuments app in NC). I believe something for it appears in the web UI in the settings area when you’ve set NC Office to use your own server (rather than the built in one) so you don’t need even need to do it from the CLI.
This will turn off certificate validation at the NC server level for the capabilities check. If things work you can then decide whether you’re comfortable with the two internal containers talking to each other without verification of certificates or not. If so, you’re done. If not, you’ll have to make adjustment to your environment so that NC is not bypassing the proxy.
I had a similar issue, running Nextcloud on an Ubuntu server with Apache, reverse proxy, and an SSL certificate for Collabora on a Docker image. The trick for me was to add port 443.
Of course, you have to replace the nextcloud domain name with yours.
sudo docker run -t -d -p 127.0.0.1:9980:9980 -e 'domain=nextcloud\\.com:443' -e 'dictionaries=de en es sv ..' -e 'TZ=Europe/Stockholm' --restart always --cap-add MKNOD --privileged collabora/code
“I’m not currently running the same setup, but if I were to run an Nginx reverse proxy,
it would look something similar.
In this case, if we are running Collabora as a Docker (Nginx) and Cloudflare on a separate Nginx server.
For Collabora, we set up Nginx as a reverse proxy to the Docker.
This setup is converted from my working Apache configuration. It’s important to use https.
This configuration is only for the reverse proxy, something to start from. Some adjustments will likely need to be made.”