A SyncBack user reported that after installing the new Windows update ‘KB4520005’ (released on October 8, 2019) on Windows Server 2012 R2, all their SyncBack profiles backing-up to FTP server failed with the error ‘522 SSL connection failed; session reuse required: see require_ssl_reuse option in vsftpd.conf man page’.
To resolve the error, you can either uninstall the Windows update ‘KB4520005’ from the system or disable ‘require_ssl_reuse’ option on the FTP server (set the parameter ‘require_ssl_reuse’ to NO in vsftpd.conf file).