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= #<Syslog::Logger:0x0000000512f7a0 @facility=8, @formatter=#<Syslog::Logger::Formatter:0x0000000512f778>, @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.