Some sites that have both IPv4 and v6 address fail to establish a connection. The error message is: "Failed to create a socket: Address family not supported by protocol" Here is one such site: distribution.xbins.org The author has made a patch(url supplied) that gives the user dialog option to turn IPv6 off. It applies cleanly against 2.0.16.
Created attachment 21338 [details, diff] gFTP IPv6 disable option patch
checked & confirmed added patch, bumped revision to 0.16-r1 to propagate. thanks for the report.
It would be nice if there was an option to disable ipv6 at compile time