After setting up a SFTP only chroot jail I had to log with Filezilla and got error:

Error:	Server unexpectedly closed network connection
Error:	Unable to connect to server

The set up was working previously so I had no idea what was going on. In those cases, just check your logs:

nano /var/log/auth.log

In my case, I found this pesky line:

fatal: bad ownership or modes for chroot directory "/srv/www/a-domain.com/public_html"

To check what chroot a user has:

nano /etc/passwd # we have a list of all users and its home directory.

The structure has to be so that the user's chroot has to be owned by root and not writable by the group.