What is causing Passenger to throw this error "Cannot disable sole enabled process in group because spawning is not allowed ..."?

Question

All of a sudden, my nginx error log is full of this error message

[ 2013-10-31 22:56:41.2124 19959/7ff254aae700 Pool2/Group.h:964 ]: Cannot disable sole enabled process in group /app/current#default because spawning is not allowed according to the current configuration options

Here /app/current is the folder in which my rails application is located (it is the Rails.root)

This error message is most likely coming from line 962 of this Phusion Passenger source file

One of my production app instance froze. I am suspecting this error is the reason.

I am not sure which configuration option is being referenced here. How do I fix this error?

This application was running fine for months now on the same setup. I just upgraded passenger to 4.0.20 last week.

Update:

I am using OOB. Here are my passenger configuration

 passenger_root /usr/local/rubies/1.9.3-p448/lib/ruby/gems/1.9.1/gems/passenger-4.0.20;
 passenger_ruby /usr/bin/ruby_tuned;
 passenger_max_pool_size 6;
 passenger_spawn_method smart-lv2;
 passenger_buffer_response on;
 passenger_min_instances 5;
 passenger_max_instances_per_app 0;
 passenger_pool_idle_time 180;
 passenger_max_requests 20;

Answer 1

You're probably using out-of-band work but with the wrong configuration. From the manual:

"Ensure that passenger_max_pool_size and passenger_min_instances are both larger than 1. Out-of-band work only works if there are at least 2 application processes."