unable to launch/connect to a remote server

Unable to use distant launch for a particular server. I have tried launching and connecting to a different remote server and it works, but just this one server, I am not able to connect. Only difference I can think of is that it requires an identity file. On just trying ```distant launch ***``` in the command line, I get:
```
Failed to launch server

Caused by:
Fatal: ssh handshake with xxxxx: [Session(-5)] Unable to exchange encryption keys
```
However using ssh to connect to the same thing works fine. Is there a way to get more logs to help debug? I have tried this command
```
distant launch ***** \
--distant-args '--log-level trace --log-file /tmp/server.log'
```

but the /temp/server.log is always empty.
```
distant launch **** --options ssh.identity_files="~/.ssh/priv_key" --distant-args '--log-level trace --log-file /tmp/server.log'
```
explicitly specifying the identity file doesnt seem to work either.