I have the same problem with this site (just tried). Log-in works, changing directories work. A simple ls doesn't.
Although you might have a good reason to download all the packages in the specific directory (different versions of the same file), you could use your browser to download a specific version of the file.
Another option that works (assuming you do want all the bz2 files in that directory): wget
The following seems to work:
wget -nd ftp://ftp.gnupg.org/gcrypt/libassuan/*bz2
The above will download all files in the directory you are standing in.
Hope this helps.