FTP is a strange beast. For a protocol that predates the web’s http, it’s amazing how often today it doesn’t work. I use Panic’s Transmit most often, and Panic do good work.
Sometimes though, Transmit fails, for no good reason. When it does, I usually revert back to the command line ftp program in the Terminal. It’s archaic, but I learnt to use it at university many moons ago and I’m glad to have the option.
If that fails too, I smell a rat. This protocol should have been perfected by now, and somehow someone’s either sent me the wrong login details or is using a non-standard server. It’s happened twice now.
So, the workaround: use the Finder. It’s basic, but it connected without any problems, instead of weird error messages. Remember the Finder next time ftp falls over.