Internal server error

Passenger encountered the following error:

The application spawner server exited unexpectedly: Connection closed

Exception class:
PhusionPassenger::ClassicRails::ApplicationSpawner::Error
Backtrace:
# File Line Location
0 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/lib/phusion_passenger/classic_rails/application_spawner.rb 139 in `spawn_application'
1 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/lib/phusion_passenger/spawn_manager.rb 223 in `spawn_rails_application'
2 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/lib/phusion_passenger/abstract_server_collection.rb 82 in `synchronize'
3 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/lib/phusion_passenger/abstract_server_collection.rb 79 in `synchronize'
4 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/lib/phusion_passenger/spawn_manager.rb 213 in `spawn_rails_application'
5 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/lib/phusion_passenger/spawn_manager.rb 132 in `spawn_application'
6 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/lib/phusion_passenger/spawn_manager.rb 275 in `handle_spawn_application'
7 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/lib/phusion_passenger/abstract_server.rb 357 in `__send__'
8 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/lib/phusion_passenger/abstract_server.rb 357 in `server_main_loop'
9 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/lib/phusion_passenger/abstract_server.rb 206 in `start_synchronously'
10 /usr/local/lib/ruby/gems/1.8/gems/passenger-3.0.19/helper-scripts/passenger-spawn-server 99