I tried again with a lower version of java (1.8.0_152), and now I have the following errors:
⇒ ./logstash-plugin install x-pack
Unhandled Java exception: java.lang.NullPointerException
java.lang.NullPointerException: null
put at java/util/Hashtable.java:460
setProperty at java/util/Properties.java:166
setProperty at java/lang/System.java:796
invoke at java/lang/reflect/Method.java:498
invokeDirectWithExceptionHandling at org/jruby/javasupport/JavaMethod.java:468
invokeStaticDirect at org/jruby/javasupport/JavaMethod.java:370
apply_env_proxy_settings at /home/cornoualis/ElasticStack6/logstash-6.0.0-rc1/lib/pluginmanager/proxy_support.rb:40
configure_proxy at /home/cornoualis/ElasticStack6/logstash-6.0.0-rc1/lib/pluginmanager/proxy_support.rb:71
at /home/cornoualis/ElasticStack6/logstash-6.0.0-rc1/lib/pluginmanager/main.rb:26
runInterpreter at org/jruby/Ruby.java:845
runInterpreter at org/jruby/Ruby.java:849
runNormally at org/jruby/Ruby.java:752
runNormally at org/jruby/Ruby.java:765
runFromMain at org/jruby/Ruby.java:578
doRunFromMain at org/jruby/Main.java:417
internalRun at org/jruby/Main.java:305
run at org/jruby/Main.java:232
main at org/jruby/Main.java:204