In our convention, ftp://cohosh/u01/app means something different: u01/app relative to the default folder (sometimes called the home folder) that the FTP site/account connects to, which is very often not the root folder.
If you look in the FTP log it may show a different folder to what you expect being CD'd into (and failing) if you omit the root slash.
That's quite different than most other FTP clients work, they address this issue by doing a PWD after connecting and then reflecting that in the URL, or by forcibly changing to the root folder when connecting.
It means I can't just click on FTP URL's (in websites, emails, etc) I will need to always copy, paste, edit, and then connect. Which makes using Directory Opus as my day to day client really annoying.