This was a total pain to locate, as the exceptions being raised were pointed to UserSession.find in Authlogic::Session::Persistence.

gems in question:

  activerecord-session_store 0.1.1
  authlogic 3.4.6
  activerecord 4.2.3
  rails 4.2.3

I noticed that the only meaningful difference between the two environments was the following instance variable in Rails.application.config:

irb> pp Rails.application.config
.
.
.
 @logger=
  #,
   @level=1>
.
.
.

Commenting out this initialization in the environment allowed the application to work again.

   config.logger = ActiveSupport::TaggedLogging.new(Syslog::Logger.new('name of log'))

Searching on “usersession syslog rails 4” results in the following issue as the third search result: Version 0.1.1 breaks Syslog::Logger setups There is a fix in master, but it doesn’t seem to have been published. I had promoted activerecord-session_store to 0.1.1 because DEPRECATION WARNING: `#quietly` is deprecated in rails-4.2.0.beta4, but it looks like locking the gem at 0.1 will work otherwise except for noisy tests.