I think the error message is pretty obvious:
On the Collabora Online server you need to install a valid SSL certificate.
Currently there is a self-signed certificate installed, which is incompatible with your ownCloud installation.
I would recommend to use Let’s Encrypt to create and renew certificates from the command line:
You will have to do something like this on the Collabora Online server:
- Make sure no webserver is listening on Port 80 & 443
- Run certbot to create a certificate for the Collabora Online server’s FQDN
- Restart the webserver stopped at 1.
- Setup a cronjob that will automatically renew the generated certificate.
Unfortunately, as you didn’t fill out the post template, I’m unable to give any more specific help, as this is hugely reliant on your setup and network.