There is nothing being recorded in the server log as the error we get occurs because no data is received as part of the server response.
The production server does not have FTP access. We can however grant you SFTP access if you are able to provide us with your public SSH key.
Alternatively we can move the site to a testing server where we do have FTP access and grant you access there.
Note we moved the to our local development server last week and error persisted, so I feel confident that if we move to a pubic testing server that it would present itself there as well. Either way works for us, just let us know if you can provide an SSH key or if you’d like it move to a different server with FTP.