Seems like everyone is running Nextcloud and Collabora from docker these days…
But if you’re like me and you like running things in good old virtual machines from source, then it’s worth knowing that Nextcloud communicates with Collabora via CURL. So, getting the two to talk to each other requires making sure your server’s SSL cert is accessible to CURL, or else it will crash silently and fail to connect (see https://curl.se/docs/sslcerts.html). This took hours of manual step debugging to figure out.
For me the final solution required placing the mycert.crt for each server in /usr/local/share/ca-certificates and then running update-ca-certificates on all virtual machines (one with Nextcloud, one with Collabora, load balancer in front) so that the certificate was properly accessible by CURL in all the right places.
If in the end you can curl collabora.mydomain.com without any errors then you can rest assured that Nextcloud and Collabora can finally connect.