It’s possible to run both Nextcloud and Collabora without direct assigned TLS certificate behind reverse proxy which terminates TLS (offloading). WOPI protocoll involves 3 parties: user client, storage system (Nextcloud) and and Office Server (Collabora) where all components need connections to each other - which makes it hard or impossible to operate without proper TLS setup. I definitely recommend you to setup proper TLS configuration and use public names for every connection…