Although you should NOT be using FTP, if you need to for some reason, like we did on one server, and some people can't upload files, this might be the issue.
We run APF firewall on some servers. A couple of servers have either ProFTP or Pure-FTP installed. Over the last couple of weeks we discovered that some people could not read or write with their FTP client when ftping into our servers.
If you experience this, check that MONOKERN is off.