@Xuo again please review working Apache config your Apache config from the first post looks more like it’s created for stand-alone collabora-online. please adopt for collabora-built-in as shown above and post fresh config and new logs.
this is a good point - likely Apache tries https because of ProxyPass directive pointing to https://
collabora-built-in should not be exposed directly with port 9980 but via richdocumentsproxy as visible from the logs: