I just ran into a bug when we installed a new gem in the same Ruby install that runs both our test environment and our Redmine install. The problem is that the new i18n gem is not compatible with rails 2.3.5.
The solution was to change this line
gem 'i18n', '>= 0.1.3'
gem '18n', '<=0.4.1'
To ensure that Redmine grabs an earlier version of the i18n gem. Hopefully this saves someone else out there from a few hours of frantic googling.