You should have both Nextcloud and Collabora having valid DNS names (which is prerequisite for TLS). All this names should exists in DNS and be resolvable by each component - client Nextcloud server and Collabora must resolve all the DNS names to the right ip (your real server ip in this case as 127.0.0.1 might be not the right one especially if you use some virtualization)
You can refer to Nextcloud forum where I posted some drawings and references: