Passenger 5.0.28 gets error trying to start Rails 3.2 application

I'm getting the following error attempting to access a new deployment of an old Rails 3.2 application:

An error occurred while starting up the preloader.
It sent an unknown response type "socket: 
http://unix:/tmp/passenger.gChlXdF/apps.s/preloader.1q3kca0\n".

I saw a similar question and answer on SO, but my situation is different. My config.ru file is as follows:

require ::File.expand_path('../config/environment',  __FILE__)
run MyApp::Application

So, not doing any STDOUT redirects there. It looks like Passenger is giving itself a bad time by the looks of the error. Not sure where to begin.