With passive FTP connections (which is the default) the FTP server opens a port which SyncBack then connects to. If that port is not open in the firewall then SyncBack cannot connect to the FTP server to transfer the data. All FTP servers allow you to define what range of ports should be used with passive connections. How that is defined depends on the FTP server software used. You should refer to the appropriate documentation for the FTP server or contact their technical support. 2BrightSparks cannot provide technical support for FTP server software.
For the Microsoft IIS FTP server refer to the following:
You must also open that same range of ports in the firewall on the computer running the FTP server software. Again, how that is done depends on the firewall software and operating system. You should refer to the appropriate documentation for the firewall server or contact their technical support. 2BrightSparks cannot provide technical support for firewall software.