I think you should better have started new thread as you issues has nothing to do with traefik. And you definitely should describe the issue…
The process of online edit documents in a browser called WOPI… search the web for you favorite explanation e.g. this one from Microsoft or this good drawing - don’t worry it’s about Sharepoint, the process is exactly the same. This official page shows some issues as well
a very basic drawing of the integration is here… followed by more complete example.
Most important requirement: each involved endpoint system needs to talk/reach others: client to both Collabora and Nextcloud, Collabora to Nextcloud and Nextcloud to Collabora. I’m not such deep into the mechanism but I could imagine you miss proper certificates as you stated you test everything internal and this could be an issue already - today integrations often require TLS which is almost impossible very hard with local setups… I would start with proper DNS/TLS setup (verify from each point - often container have another DNS view than the host or client) following with careful log reading…