Sometimes the Cmder SSH agent seems to be borked and anything you try to connect to just rejects. I often run into this when running an Ansible command on a remote host
ssh root@xx.xxx.xxx.xxx
root@xx.xxx.xxx.xxx: Permission denied (publickey).
ssh_exchange_identification: Connection closed by remote host
Fix
Just kill the ssh-agent.exe
processes (Task Manager works just fine),
Then restart the SSH agent,
$ start-ssh-agent
Removing old ssh-agent sockets
Starting ssh-agent: done
This does not work with the latest versions of Cmder.