I run into the same issue with nginx without any Cloudfare rules set for the forum and without any CDN active, so the webserver serves content directly.
Fix:
NodeBB
config.json
There should be no forward slash at the end which complies with the documentation:
"url": "https://blabla.com/forum",nginx.conf
here note the double slash on the first line /forum/
The double slash solves the issue, at least during initial testing on different browsers and devices.
Not sure if this is a Windows/nginx specific thing as the path's are set a bit different.