For me it sounds there is an issue with Collabora CODE running native on the host. AFAIK best place to ask for support in such case is Collabora Forum.
this sounds your proxy can not access CODE backend (which makes no sense as you state /hosting/discovery shows XML content). Look at Collabora integration guide it will help you to understand mechanics of CODE integration.
- I see your proxy is configured to use https:// and connect to :9980 of CODE. usually this port is plan http
- you stated CODE runs on a different system - in this case 127.0.01 is wrong: https://127.0.0.1:9980/hosting/capabilities
P.S. and last advice unrelated to this problem: NC26 is out of support since 2024-03, please upgrade to a supported version.