If anyone ist facing the problem, opening documents in collabora get’s a 404 although the office server works:
Try to control you reverse proxy configuration.
They changed the entrypoints as describes here: https://sdk.collaboraonline.com/docs/installation/Proxy_settings.html#reverse-proxy-with-nginx-webserver
From loaleafled to browser and from lool to cool - whyever.
Took me about an hour to find it out. But It’s easy to fix.