(Using the latest 3-0-stable and rvm, and ruby-1.9.2-p0)
Yesterday everything worked, my own changes are very minor and in the application only. Did anything change that could have caused this problem? Unfortunately simply going back in rails alone to the original "RC2" git-tag did not immediately help, so it may be some other problem (on my own system only, even if I'm convinced I didn't change anything significant since yesterday except for running "bundle update" once too often, maybe).
mha@ubuntu-vm:~/Letexa$ rails server => Booting WEBrick => Rails 3.0.0.rc2 application starting in development on http://0.0.0.0:3000 => Call with -d to detach => Ctrl-C to shutdown server Exiting /home/mha/.rvm/rubies/ruby-1.9.2-p0/lib/ruby/1.9.1/json/common.rb: 27:in `remove_const': constant JSON::Parser not defined (NameError) from /home/mha/.rvm/rubies/ruby-1.9.2-p0/lib/ruby/1.9.1/json/ common.rb:27:in `parser=' from /home/mha/.rvm/rubies/ruby-1.9.2-p0/lib/ruby/1.9.1/json/ext.rb: 10:in `<module:Ext>' from /home/mha/.rvm/rubies/ruby-1.9.2-p0/lib/ruby/1.9.1/json/ext.rb: 6:in `<module:JSON>' from /home/mha/.rvm/rubies/ruby-1.9.2-p0/lib/ruby/1.9.1/json/ext.rb: 3:in `<top (required)>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `block in require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `block in load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:591:in `new_constants_in' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/rubies/ruby-1.9.2-p0/lib/ruby/1.9.1/json.rb:6:in `<module:JSON>' from /home/mha/.rvm/rubies/ruby-1.9.2-p0/lib/ruby/1.9.1/json.rb:2:in `<top (required)>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `block in require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `block in load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:591:in `new_constants_in' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/core_ext/object/to_json.rb:3:in `<top (required)>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `block in require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `block in load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:591:in `new_constants_in' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/core_ext/object.rb:10:in `<top (required)>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `block in require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `block in load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:591:in `new_constants_in' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/core_ext.rb:2:in `block in <top (required)>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/core_ext.rb:1:in `each' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/core_ext.rb:1:in `<top (required)>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `block in require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `block in load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:591:in `new_constants_in' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/all.rb:3:in `<top (required)>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `block in require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `block in load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:591:in `new_constants_in' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/application/bootstrap.rb:16:in `block in <module:Bootstrap>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/initializable.rb:25:in `instance_exec' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/initializable.rb:25:in `run' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/initializable.rb:50:in `block in run_initializers' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/initializable.rb:49:in `each' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/initializable.rb:49:in `run_initializers' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/application.rb:134:in `initialize!' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/application.rb:77:in `method_missing' from /home/mha/Letexa/config/environment.rb:6:in `<top (required)>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `block in require' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `block in load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:591:in `new_constants_in' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:225:in `load_dependency' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ activesupport/lib/active_support/dependencies.rb:239:in `require' from /home/mha/Letexa/config.ru:3:in `block in <main>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/gems/rack-1.2.1/lib/rack/ builder.rb:46:in `instance_eval' from /home/mha/.rvm/gems/ruby-1.9.2-p0/gems/rack-1.2.1/lib/rack/ builder.rb:46:in `initialize' from /home/mha/Letexa/config.ru:1:in `new' from /home/mha/Letexa/config.ru:1:in `<main>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/gems/rack-1.2.1/lib/rack/ builder.rb:35:in `eval' from /home/mha/.rvm/gems/ruby-1.9.2-p0/gems/rack-1.2.1/lib/rack/ builder.rb:35:in `parse_file' from /home/mha/.rvm/gems/ruby-1.9.2-p0/gems/rack-1.2.1/lib/rack/ server.rb:162:in `app' from /home/mha/.rvm/gems/ruby-1.9.2-p0/gems/rack-1.2.1/lib/rack/ server.rb:248:in `wrapped_app' from /home/mha/.rvm/gems/ruby-1.9.2-p0/gems/rack-1.2.1/lib/rack/ server.rb:213:in `start' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/commands/server.rb:65:in `start' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/commands.rb:30:in `block in <top (required)>' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/commands.rb:27:in `tap' from /home/mha/.rvm/gems/ruby-1.9.2-p0/bundler/gems/rails-9df512b/ railties/lib/rails/commands.rb:27:in `<top (required)>' from script/rails:9:in `require' from script/rails:9:in `<main>'