This is for you if Passenger gives you the following useless error message.

Passenger encountered the following error:\
The application spawner server exited unexpectedly: Unexpected end-of-file detected.

Exception class:
: PhusionPassenger::Rack::ApplicationSpawner::Error

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Most often this happens because you are missing a gem. Usually Passenger would tell you about that but in some cases it can't.

To resolve this issue, run:

bundle install

If this does not do the trick for you, take a look at the Apache log files for details.