Hi Mark,
For the correct upload backups to FTP, we use Rebex libraries. To resolve this issue we have contacted their support team Data connection cannot be opened with this PROT setting (521) - Rebex Q&A Forum and for further investigation, they request additional information.
Could you please enable the advanced log with “Trace” level, reproduce the issue and send logs to our dev team. Here are more details on how to do it How to send Log to developers | SQLBackupAndFTP's blog
Note! With the “Trace” level, a log can contain your passwords. Before sending a log to dev team please open it using any text editor and remove the information you prefer to keep in secret. You can find them by clicking the “Open folder” link at the “Advanced Log” window (“Tools” > “Advanced Log…”). We plan to send these logs to Rebex’s support team.
Please let us know if the logs will be sent, also please provide us with your application id (“Help” > “About”).
Sorry for the inconvenience.