puma: "Bad response from server: 500" on restart during Capistrano puma:restart

I’m on puma 2.0.1 behind nginx, with Rails 3.2.12, Ruby 2.0.0, server is Ubuntu Precise

Each time I try to deploy my app via Capistrano, at the deploy:restart hook, or when I try a restart through cap puma:restart I constantly get this:

  * 2013-05-01 11:24:26 executing `puma:restart'
  * executing "cd /var/www/project/current && bundle exec pumactl -S /var/www/project/shared/sockets/puma.state restart"
    servers: ["staging.example.com"]
    [staging.example.com] executing command
 ** [out :: staging.example.com] Bad response from server: 500

The puma server gets restarted correctly but the Capistrano deployment process gets interrupted.

This also happens when I try to stop it:

 * executing "cd /var/www/project/current && bundle exec pumactl -S /var/www/project/shared/sockets/puma.state stop"
    servers: ["staging.example.com"]
    [staging.example.com] executing command
 ** [out :: staging.example.com] Bad response from server: 500