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.
There should be no forward slash at the end which complies with the documentation:"url": "https://blabla.com/forum",
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.