This post has been viewed more than 400 times, so I'm not the only one to have lost time on a problem that should not exist.
To solve, create an FTP account on the user, but another pb come, you can not create a folder at the same level as public_html, practical in many cases...
The solution back to the first reflex, connect FTP directly to the user account and then, like a miracle the connection is established without pb. After, you can then connect to any user without the need to create additional FTP account.
I thank the developers for this interface, but do not try to compete with Cpanel or Plesk which I think are a little too heavy to manage sites. Facilitate security, implementation and daily functioning as backups and more jobs is a general need.
Thanks
Francois