Any news here?
I have the same issue connecting to an non standard ftp port using WP 4.3.1 and BackWpUp 3.2.1.
Connecting via ftp port 2021 are failing using IP or DNS name. Using another ftp tool works fine on port 2021 in both passive and non passive mode.
Changing the port on the ftp server I would like to connect to, to port 21 it works just fine.
Therefore it has to be related to the BackWpUp plugin.
Hope someone are able to help.
Thanks in advance.
