The modeprobe nf_conntrack_ftp doesn't output any messge or error? Not sure what it is suppose to output.
I did a test from a machine to the server running the vsftp server and using ncftp or ncftpls, but in the past have also used ftp with the same results.
With the line disabled everything seems to work, but without it seems to fail, but in one section changed passive mode, back it seemed to continue??
These machines are in the same 192.168.7.x network connected to the same switch? All are running Fedora 24, upgraded via dnf from 23 over the summer. With the 23, never had any issues.