Some FTP clients will not allow changing to a directory if the path does not ends with a slash. For example, try out this in a public FTP:

from ftplib import FTP
ftp = FTP('ftp.unicamp.br')
ftp.login()
ftp.cwd('pub/libreoffice') # throws error
ftp.cwd('pub/libreoffice/') # OK

The problem is urllib.request doesn't include the trailing slash, thus throwing an error. This behavior also happens with the command line ftp client.

I think this happens because the libreoffice directory is a symlink, and this can be a FTP server specific behavior.