Output the hostname of the upload destination to the Box Edit log
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.