See these articles for possible help:

"Folder Error" error message when you attempt to gain access to FTP server
http://support.microsoft.com/kb/260156/en-us

"The Page Cannot Be Displayed" Error Message Occurs When You Try to Access
an FTP Site
http://support.microsoft.com/kb/328785/en-us

If no joy, make sure you haven't changed anything in regards to your proxy
settings in IE.
Also make sure there's been no change made to the "User Authentication"
setting at Tools...Internet Options...Internet Zone...Custom button (way at
the bottom).

If still no joy, the check the settings on the software used on the FTP
server.