Foreman Passengar Error while starting up preloader

I am installing Foreman 1.6. After installation,
/etc/httpd/conf.d/errro.log shows:

[Pool2/Implementation.cpp:849 ]: Could not spawn process for group
/usr/share/foreman#default: An error occured while starting up the
preloader.
in 'void
assenger::ApplicationPool2::SmartSpawner::handleErrorResponse(Passenger::ApplicationPool2::SmartSpawner::StartupDetails&)'
(SmartSpawner.h:455)
in 'std::string
Passenger::ApplicationPool2::SmartSpawner::negotiatePreloaderStartup(Passenger::ApplicationPool2::SmartSpawner::StartupDetails&)'
(SmartSpawner.h:558)
in 'void Passenger::ApplicationPool2::SmartSpawner::startPreloader()'
(SmartSpawner.h:206)
in 'virtual Passenger::ApplicationPool2::ProcessPtr
Passenger::ApplicationPool2::SmartSpawner::spawn(const
Passenger::ApplicationPool2::Options&)' (SmartSpawner.h:744)
in 'void Passenger::ApplicationPool2::Group::spawnThreadRealMain(const
Passenger::ApplicationPool2::SpawnerPtr&, const
Passenger::ApplicationPool2::Options&, unsigned int)'
(Implementation.cpp:782)

[ agents/HelperAgent/RequestHandler.h:1971 ]: [Client 20] Cannot checkout
session. An error occured while starting up the preloader.

Any clue why there is error to start preloader?

Thanks a lot!