Collabora does not work in snap nextcloud behind Nginx

I installed nextcloud via snap package and run it behind a simple Nginx proxy with a config very similar to this:

Just on port 80.
SSL done via Cloudflare.

Everything works just fine except the Office with built-in CODE server.
It tries to open a document, but the “loading wheel” stays forever.

Screenshot from 2023-01-01 22-22-04

No errors in JS console, no network error…

Any clues what to do?

Thank you!