Uninitialized constant ActiveSupport::Dependencies::Mutex (NameError)
In case you can't upgrade to Ruby on Rails 2.3.11 (and to expand on douglasr's answer), thread must be required at the top of boot.rb. For example:
require 'thread'# Don't change this file!# Configure your app in config/environment.rb and config/environments/*.rb...
I was able to fix this by downgrading RubyGems to 1.5.3, since it happens with RubyGems 1.6.0+ and Rails < 2.3.11:
gem update --system 1.5.3If you had previously downgraded to an even earlier version and want to update to 1.5.3, you might get the following when trying to run that:
Updating RubyGemsERROR: While executing gem ... (RuntimeError) No gem names are allowed with the --system optionIf you get that error, then update, so that it lets you specify the version, and then downgrade again:
gem update --systemgem update --system 1.5.3
This is an incompatibility between Rails 2.3.8 and recent versions of RubyGems. Upgrade to the latest 2.3 version (2.3.11 as of today).