Found it…when I looked at the docker start command and all attributions I noticed it needed to have the domain of the nextcloud instance.
Add this: to the WOPI part:
<host desc="Regex pattern of hostname to allow or deny." allow="true">share\.domain\.com</host>
make sure to escape the dots, single backslash this time… fill in the domain of your nextcloud instance…
Now I am only left with some faulty “tools” in the toolbar of collabora… Will post something new for that.