are any of your sites on port 21?
it's possible that your router is doing NAT, so when it sees start of FXP transfer it thinks it's regular ftp download and changes PORT command to have your ip address...
what you can do is to look at your router settings and see if you can disable ftp "fixing"
feature.
or play around with Alternative FXP options so that site which is NOT on port 21 would get PORT command, and site on port 21 would get PASV command.
or see if you can connect to those sites using ssl encryption so that your router wouldn't be able to monitor ftp commands and consequently "fix" them.