I’m sorry I don’t understand this request. maybe you anonymized it too hard but this not what I expect to see. (and I’m even more confused with a screenshot above - it show “not built-in” Collabora - I didn’t notice it before) - please explain you setup in more details.
likely this is related to the security limitation of the Collabora (aliasgroup setting for stand-alone CODE) - the system tries to check if the IP address of the accessing system belongs to the allowed DNS records - and this fails for this reason access is denied. I’m under impression the IP is the public IP of your server. Take a look at this post it describes how to make different Docker containers talk directly to the reverse proxy rather going through the internet.