Possible reasons:
1. Piped logging seems to have caused a problem that’s been fixed in version 4.0.6 of Passenger
2. If you need to debug Passenger problems, use this option:
PassengerDebugLogFile <path to log>
and this to specify the log level:
PassengerLogLevel <integer – see guide>
(in Apache syntax)
passenger_log_level
(in Nginx syntax)
Apache Guide:
http://www.modrails.com/documentation/Users%20guide%20Apache.html#_logging_and_debugging_options
Nginx Guide: