3172798 created the issue

Directory listings fail to display in the Cyberduck window if ProFTPd is configured to show file sizes in human-readable format; i.e., ListOptions "-lh". Reverting to "-l" in ProFTPd config fixes the problem. Directories are shown correctly in command-line FTP and in other apps when the "-h" (human-readable) option is used. Seems to be a problem with the way incoming directory listing information is interpreted for the UI prior to display. In cases where the first elements listed in a directory have a byte-sized entry, they display; any files with K or M or G size entries do not.