HTTP 400 Bad Request - Embedded Device

Thanks for the additional information, and clarifying that you were referring to appending to an upload session. 

Anyway, regardless of whether you're using the single /2/files/upload endpoint, or the upload sessions endpoints, the Dropbox API should only be sending one response per request. (I also haven't heard any other reports like this.)

Unfortunately, Dropbox doesn't offer a way to retrieve error logs for an app.

Looking at the actual headers/content of the second response you showed in your first message, it looks like that may be the generic 400 error that the Dropbox sends when a request is malformed and couldn't be understood. (It's a generic HTML error page, not specific to the API.) That does seem to support the idea that the client is sending some other request, perhaps a corrupted or partial request, that can't be interpreted as a valid API call. For example, perhaps the client is sending more data over the socket after the actual intended HTTP message for the API call, and that extra data is not properly formed as an HTTP message, resulting in the error.