I have not. Note on what you are trying, the Next Hop Proxies only support HTTP (from what I remember).
You probably need to disable the "Configure Upstream Server FTP" rule and add an event to "URL rewrite" saying URL.Host = 10.x.x.x
thanks for the hint! I've re-configured the rule according to your post. But unfortunately it doesn't solve my problem.
I also traced packets at MWG and Backend-FTP-Server and saw that no packets do arrive at the FTP-Server. On the other hand MWG receives packets from the client. The client does get authenticated by MWG properly, but obviously MWG does not open a connection to the FTP-Server to pass on the creditentials.
According to the "Rule tracing central" feature the rule (see screenshot) gets executed correctly. So, either there are some more properties needed or in the configuration other settings are required.
Have you ever tested or implemented this feature?