If you have access to the FTP server then you can try and force the max number of connections to be unlimited (by default it is limited to 50 connections)
With VSFTPD you add
into the /etc/vsftpd.conf file and restart the service.
However, I am not sure that this will fix your problem. I have had the same issue with oXygen and the only thing that works is usually to kill the tcp connections manually. This gets pretty frustrating but will work:
sudo tcpkill -9 -i <network-interface> host <host-of-your-ftp>
So for example if you had your network interface at eth0 and you were connecting to example.com
sudo tcpkill -9 -i eth0 host example.com
To get a list of your network interfaces you can do
Typically its eth0 for hard wired and wlan0 for wireless bit you should double check.
Hope this helps. If you find another solution to this please let me know.