Incorrect indentation in auth_config.yml can diable users from logging in to Mingle.

Problem Statement: Users are presented with the "Mingle encountered an error" page upon Mingle restart usually after a change has been done to auth_config.yml file located under the Mingle config directory.

Log extract:

Processing ProfileController#login (for 127.0.0.1 at 2011-09-08 17:08:46) [GET]

[2011-09-08 17:08:46,700] [btpool0-4] [/]   Parameters: {"controller"=>"profile", "action"=>"login"}

[2011-09-08 17:08:46,719] [btpool0-4] [/]
NoMethodError (undefined method `is_external_authenticator?' for nil:NilClass):
  lib/authenticator.rb:94:in `using_external_authenticator?'
  lib/authenticator.rb:83:in `invalidate_session_external_authentication_if_not_using_external_authenticator'

Solution: This problem is usually caused due to incorrect indentation during modification to auth_config.yml file. An example:

basic_authentication_enabled:true

Correct syntax: basic_authentication_enabled: true

Please refer to the following documentation before making any changes to the auth_config.yml file: http://www.thoughtworks-studios.com/mingle/3.3.1/help/mingle_ldap_authentication.html