It indeed looks like a local firewall or NAT problem of the client.
See https://winscp.net/eng/docs/ftp_modes#smart
See https://winscp.net/eng/docs/ftp_modes#smart
Before posting, please read how to report bug or request support effectively.
Bug reports without an attached log file are usually useless.
. 2017-12-27 12:36:14.492 Host name: xx.xxx.xxx.xxx (Port: XXXX)
. 2017-12-27 12:36:15.305 Connecting to xx.xxx.xxx.xxx:XXXX ...