You may encounter problems with passenger starting an application with an updated rails.
If you find an error like this in the apache error log:

[ 2015-08-21 10:53:04.1266 17680/7f4909bf7700 Pool2/Implementation.cpp:883 ]: Could not spawn process for group /var/www/example.com/current#default: An error occured while starting up the preloader.
     in 'void Passenger::ApplicationPool2::SmartSpawner::handleErrorResponse(Passenger::ApplicationPool2::SmartSpawner::StartupDetails&)' (SmartSpawner.h:455)
     in 'std::string Passenger::ApplicationPool2::SmartSpawner::negotiatePreloaderStartup(Passenger::ApplicationPool2::SmartSpawner::StartupDetails&)' (SmartSpawner.h:566)
     in 'void Passenger::ApplicationPool2::SmartSpawner::startPreloader()' (SmartSpawner.h:206)
     in 'virtual Passenger::ApplicationPool2::ProcessPtr Passenger::ApplicationPool2::SmartSpawner::spawn(const Passenger::ApplicationPool2::Options&)' (SmartSpawner.h:752)
     in 'void Passenger::ApplicationPool2::Group::spawnThreadRealMain(const SpawnerPtr&, const Passenger::ApplicationPool2::Options&, unsigned int)' (Implementation.cpp:804)

[ 2015-08-21 10:53:04.1273 17680/7f4907ae7700 agents/HelperAgent/RequestHandler.h:2210 ]: [Client 19] Cannot checkout session.
Error page:
This application is a Rails 3 application, but it was wrongly detected as a Rails 1 or Rails 2 application. This is probably a bug in Phusion Passenger, so please report it.

!> Error

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You have to restart the apache webserver. It seems that passenger does not apply all new settings/informations about the application correctly if you just reload apache or restart the passenger app.