Maybe FTP server is blocking new transfers from TRB. You should check FTP server access list for allowed IP addresses.
By default, TRB140 is not filtering or blocking any outgoing traffic.
I recommend you to check does router use latest available firmware version? If not please update routers firmware to the latest one:
Furthermore, please navigate to WebUI -> System -> Administration -> Troubleshoot -> Enable TCP dump. After enabling it, try to connect to a FTP server and transfer some data. After couple of try, download TCPdump with the Troubleshoot file and PM them to me.
Note: after that DO NOT forget to disable TCPdump.