At 500mb Chrome gives “network error”, Firefox mentions “check your internet connection” so it’s not a browser issue.
Really don’t know how to fix this, it was working fine for some months until this started happening. Backblaze support mentions it’s working ok on their side and downloading the file directly from my bucket works correctly.
It sounds like you are currently reaching the cacheable file size limit in Cloudflare, which is causing the issue you are reporting.
You could try creating a cache rule that sets the requests that go to the /download/ path to bypass the Cloudflare cache and see if that fixes the issue you are having.
The file you linked above is not using the files subdomain, but www. But your Rule is for the files subdomain.
Apart from that, you might be running into problems with the Cloudflare ToS. You are generally not allowed to use the proxy for file distribution, unless you are an Enterprise customer. Are downloads also that slow (~400kbit/s) without using Cloudflare? If not, I’d assume that you are already limited for ToS violations.