hi @wario sorry I missed the thread till now.
from your posts I have the feeling you already follow this guide, if not please double check Nextcloud Collabora integration
for me it sounds this is the culprit:
CODE fails to access the file stored in NC. If you still have this issue focus on it - could be something silly like TLS issue… (but if curl https://nextcloud.example.com/status.php works this should not be the case…)…
I have the feeling you are close to the solution - please double and triple check all steps and likely it starts working! if not post all the configs and fresh logs (NC, CODE and client)
I’m sorry you are fundamentally wrong - the guide I referenced above was created from docker-only install (one CODE and two NC instances run good since 3 years and 6-7 major updates).