it really could be anything.
did you try asking those people that got errors, what Operating System they were using, the version, the Internet Browser used and that version. Maybe their internet connection.
what kind of server are you using?
also, did you check that the directory does exist?
the question, unfortunately is quite vague. if you could tell us somethign more specific like:
when a person clicks on this, then they get the error, but not on that.
when a person is on WinXP, using IE 5.5 they get the error.
etc....