Onlyoffice integration in Nextcloud - X-Frame-Options trouble

Hi all I’m running a small home NAS on FreeNAS 11.2 U7. For a long time I’m trying to integrate an Onlyoffice document-server into Nextcloud. Finally, with RancherOS I managed to successfully install an Onlyoffice docker image. The testpage with the green checkmark shows up for port 80 as wells as…