Box Edit selects the upload destination host from the preflight response, and the upload destination is vary.
If the customer has a strict set of FW rules, only certain files may fail to upload, in which case you will need to get a network capture to verify the destination host.
Obtaining network captures is a difficult operation for endusers to perform, and it is taking a long time to resolve the issue.
Please consider outputting the selected hostname to the log.