Keep back up
of your squid.conf and use mine (attached file). Restart squid and try again.
May be I am wrong and just wasting your time but here no body is assisting you except me that's why you have to bear me.
You have said that outside LAN, you can connect to same ftp and only difference between LAN's system and outside LAN's is squid proxy.
That's why I am thinking the reason is squid.
Most of the times users play with system and solve their problems themselves. May be you can do it too after all you are the person who has access.