BadaBing

We have FTP access to a specific webserver. The puzzle is that some stations can access the server using Explorer and some can't. Those that can't, can't access through the command line either. But most puzzling, the server that connects to the router can't access either. The errors refer to the host using Squid and an authentication problem, and I could understand if it was nothing able to access it, but it being so hit-or-miss is puzzling me. Any ideas?