I haven't been able to connect to an FTP server from Finder, if I use other client there is no problem.
The username I am using to connect is in the form
domain part in the user name is not related to the ftp server I am trying to connect to, is just that the user name contains an '@'.
I found a similar issue where the password contains '@', in my case it is in the username.
found that any password with an '@' symbol in it caused Finder to immediately display that error and not even try to send any traffic to the FTP server.
For real, is Finder unable to connect to FTP servers if there are '@' in the credentials?
Host is specified here
Then credentials are specified here
It is a common and very simple use case, so it is hard to believe that Finder, the default file explorer doesn't support it.