Can't connect Robo3T to Google Cloud Platform compute engine via SSH

I can SSH into my instance from the terminal like this:

ssh -i <path_to_private_key> <username>@<external_ip_address>

But I can not connect Robo3T via an SSH tunnel, using the same credentials - I get the error:

Resource temporarily unavailable. Authentication by key (/home/alex/.ssh/id_rsa) failed (Error -16). (Error #11)

Has anyone managed this, and if so, how can I?