Capistrano 3 SSHKit::Runner::ExecuteError: Exception while executing on host [hostname ]agent could not sign data with requested identity
I solved a similar issue by just issuing ssh-add. It seems that my current environment hasn't properly picked up the keys and readding them fixed the issue.
I had the same error.
ssh-copy-id user@ipaddressHelped me to solve this.
I had the same issue but in my case I had to delete file .ssh/known_hosts from my local machine.