Redmine

Same problem, but I use 0.9.3 from Ubuntu repos. I solved it differently - rearranging the code to try each returned entry until successful.
The new code in app/models/auth_source_ldap.rb looks like this:

  def authenticate(login, password)
    return nil if login.blank? || password.blank?
    attrs = []
    # get user's DN
    ldap_con = initialize_ldap_con(self.account, self.account_password)
    login_filter = Net::LDAP::Filter.eq( self.attr_login, login ) 
    object_filter = Net::LDAP::Filter.eq( "objectClass", "*" ) 
    dn = String.new
    ldap_con.search( :base => self.base_dn, 
                     :filter => object_filter & login_filter, 
                     # only ask for the DN if on-the-fly registration is disabled
                     :attributes=> (onthefly_register? ? ['dn', self.attr_firstname, self.attr_lastname, self.attr_mail] : ['dn'])) do |entry|
      dn = entry.dn
# IVA2K fix for OpenLDAP:
      unless dn.empty?
        attrs = [:firstname => AuthSourceLdap.get_attr(entry, self.attr_firstname),
                 :lastname => AuthSourceLdap.get_attr(entry, self.attr_lastname),
                 :mail => AuthSourceLdap.get_attr(entry, self.attr_mail),
                 :auth_source_id => self.id ] if onthefly_register?
        logger.debug "DN found for #{login}: #{dn}" if logger && logger.debug?
        # authenticate user
        ldap_con = initialize_ldap_con(dn, password)
        if ldap_con.bind
          # return user's attributes
          logger.debug "Authentication successful for '#{login}'" if logger && logger.debug?
          return attrs
        end
      end
    end
    return nil

  rescue  Net::LDAP::LdapError => text
    raise "LdapError: " + text
  end