Could be, but if you haven't changed anything I don't see why it would be your FTP client.
I've never used Publisher for websites, but if everything works fine locally (looking at the site on your hard drive), then you could always try using FileZilla (assuming you're on windows):
If I ever use Windows for FTP stuff I use FileZilla; it's about the best there is and is totally free. Worth a try with that.
http://filezilla.sourceforge.net/
Otherwise, your host could have changed some settings? But since you mentioned that you've tried other servers, sounds unlikely.
Is there anything you've changed that you think may have caused it, since you last uploaded successfully?