Somone else posted in these forums about a week and half ago, with a very similar problem. Their upload would stop at about 50mb. They discovered some method to work around it - you should be able to search for that post.
Enless there's some bug in the software that you're using, I don't think it's a user error. If you started the download, and it was going, enless the FTP program is locking up, or you're canceling the download... it's probably not user error.
I'd run a few test personally, see if once in a while the upload will succeed, or it never works. If you have access, try uploading the file to a different server. If you can narrow down where the problem is, it will help fix it. If you can narrow it down to DH server only (or not) send support your findings and ask them to check into it. There may well be a misconfiguration on your server.