I didn't have access to the server in question but I managed to set it up locally and could reproduce the problem:
undefined method `empty?' for nil:NilClass
C:/sonarqube-5.6.5/web/WEB-INF/lib/authenticated_system.rb:132:in `login_from_basic_auth'
org/jruby/RubyProc.java:290:in `call'
org/jruby/RubyProc.java:224:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/http_authentication.rb:126:in `authenticate'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/http_authentication.rb:116:in `authenticate_with_http_basic'
C:/sonarqube-5.6.5/web/WEB-INF/lib/authenticated_system.rb:129:in `login_from_basic_auth'
C:/sonarqube-5.6.5/web/WEB-INF/lib/authenticated_system.rb:11:in `current_user'
C:/sonarqube-5.6.5/web/WEB-INF/app/controllers/application_controller.rb:102:in `set_user_session'
org/jruby/RubyKernel.java:2223:in `send'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/activesupport-2.3.15/lib/active_support/callbacks.rb:178:in `evaluate_method'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/activesupport-2.3.15/lib/active_support/callbacks.rb:166:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/filters.rb:225:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/filters.rb:629:in `run_before_filters'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/filters.rb:615:in `call_filters'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/filters.rb:610:in `perform_action_with_filters'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/benchmarking.rb:68:in `perform_action_with_benchmark'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/activesupport-2.3.15/lib/active_support/core_ext/benchmark.rb:17:in `ms'
jar:file:/C:/sonarqube-5.6.5/lib/server/jruby-complete-1.7.9.jar!/META-INF/jruby.home/lib/ruby/1.8/benchmark.rb:308:in `realtime'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/activesupport-2.3.15/lib/active_support/core_ext/benchmark.rb:17:in `ms'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/benchmarking.rb:68:in `perform_action_with_benchmark'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/rescue.rb:160:in `perform_action_with_rescue'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/flash.rb:151:in `perform_action_with_flash'
org/jruby/RubyKernel.java:2223:in `send'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/base.rb:532:in `process'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/filters.rb:606:in `process_with_filters'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/base.rb:391:in `process'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/base.rb:386:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/routing/route_set.rb:450:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/dispatcher.rb:87:in `dispatch'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/dispatcher.rb:85:in `dispatch'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/dispatcher.rb:121:in `_call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/dispatcher.rb:130:in `build_middleware_stack'
org/jruby/RubyProc.java:290:in `call'
org/jruby/RubyProc.java:224:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/activerecord-2.3.15/lib/active_record/query_cache.rb:29:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/activerecord-2.3.15/lib/active_record/connection_adapters/abstract/query_cache.rb:34:in `cache'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/activerecord-2.3.15/lib/active_record/query_cache.rb:9:in `cache'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/activerecord-2.3.15/lib/active_record/query_cache.rb:28:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/activerecord-2.3.15/lib/active_record/connection_adapters/abstract/connection_pool.rb:361:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/config/environment.rb:67:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/string_coercion.rb:25:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/rack-1.1.6/lib/rack/head.rb:9:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/rack-1.1.6/lib/rack/methodoverride.rb:24:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/params_parser.rb:15:in `call'
file:/C:/sonarqube-5.6.5/lib/server/jruby-rack-1.1.13.2.jar!/jruby/rack/session_store.rb:70:in `context'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/rack-1.1.6/lib/rack/session/abstract/id.rb:58:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/failsafe.rb:26:in `call'
C:/sonarqube-5.6.5/web/WEB-INF/gems/gems/actionpack-2.3.15/lib/action_controller/dispatcher.rb:106:in `call'
file:/C:/sonarqube-5.6.5/lib/server/jruby-rack-1.1.13.2.jar!/rack/adapter/rails.rb:34:in `serve_rails'
file:/C:/sonarqube-5.6.5/lib/server/jruby-rack-1.1.13.2.jar!/rack/adapter/rails.rb:39:in `call'
file:/C:/sonarqube-5.6.5/lib/server/jruby-rack-1.1.13.2.jar!/rack/handler/servlet.rb:22:in `call'
/ Alix