SSH Proxy may not recognize a private key generated with recent tools, after all SSH Proxy hasn't been updated in 5 years.
Here I answer you another question asked here:
Core Tunnel may incorporates SSH Proxy features in future release, we're still exploring the feasibility of merging. Since they have different goals, so simply combine both features can make design conflicts.
I'd recommend you use Core Tunnel and create a dynamic port forwarding instead at present.
Yang