host.fqdn.tld: channel 0: new [client-session]
host.fqdn.tld: Requesting channel_open for channel 0.
host.fqdn.tld: channel 0: open confirm rwindow 32768 rmax 16384
host.fqdn.tld: channel 1: new [client-session]
host.fqdn.tld: Requesting channel_open for channel 1.
host.fqdn.tld: Channel open failure: 1: reason 4: Server supports one session per SSH client

##</code><code>##

$  ssh -v localhost -s sftp
...
debug1: Authentication succeeded (publickey).
debug1: channel 0: new [client-session]
debug1: Requesting no-more-sessions@openssh.com
debug1: Entering interactive session.
debug1: Sending environment.
debug1: Sending env LC_MESSAGES = en_AG.utf8
debug1: Sending env LANG = en_US.UTF-8
debug1: Sending subsystem: sftp