FTP is going to use a control and data port.
rainofkayos@animal ~  % getent services ftp ftp-data
In FTP's Passive mode the client side initiates the communication to by default port 20. In which case I believe there is a statement you add to enable passive transfers via squid which is something like 'ftp_passive on'. (http://www.squid-cache.org/Doc/config/ftp_passive/
). Another thing to note which I have faced in the past is what is actually resetting the connection? Is it squid resetting this connection or could it be a NAT Firewall which this communication traverses?