What does it mean?

You want to upload files to FTP sites, and the site administrators do not want to see you. Or, you use the wrong syntax when trying to file. Or the site is currently too busy to deal with your upload.

What can you do about it?

First of all, check you are using the correct syntax. And then try again later. If the problem persists, please send an e-mail to the network administrator, and ask you how to upload files to the site.