files/download giving 400 with an HTML page body from one host but not another

Thanks for the detailed report! Unfortunately, the request ID isn't enough to see what the issue is in this case.

With issues like this, where an API call receives an HTML response, it's generally because the HTTP request for the API call was mangled such that the Dropbox API servers weren't able to recognize it as an API request.

There are a few ways for that to happen, and I suspect it's related to the different versions of curl in use here.

To help debug this, can you print out the raw HTTP request for the failing call, similar to how you shared the raw response? Be sure to redact the access token of course, but please include everything else.