1) add a postbuildscript to postbuild section
2) Add build step => pick 'Execute Ruby script'
3) add some ruby code as seen below
4) click save => leads to Status Code 500

Status Code: 500
Exception: java.lang.RuntimeException: Failed to instantiate class org.jenkinsci.plugins.postbuildscript.PostBuildScript from {"buildStep":

{"kind":"hudson.plugins.ruby.Ruby","ruby":"require 'net/ssh'\n\nNet::SSH.start('aaa.bbb.ccc.ddd', '********', :password => \"********\") do |ssh|\n\tstdout = \"\"\n\tssh.exec!(\"tail -n100 /logs/xyz.log\") do |channel, stream, data|\n\t\tstdout << data if stream == :stdout\n\tend\n \n\trecapp3file = File.new(ENV['WORKSPACE']+\"/philips/results/dms_recapp_3_tail.txt\", \"w\")\n\trecapp3file.write(stdout)\n\trecapp3file.close\nend\n\nNet::SSH.start('aaa.bbb.ccc.ddd', '********', :password => \"********\") do |ssh|\n\tstdout = \"\"\n\tssh.exec!(\"tail -n100 /logs/xyz.log\") do |channel, stream, data|\n\t\tstdout << data if stream == :stdout\n\tend\n \n\trecapp4file = File.new(ENV['WORKSPACE']+\"/philips/results/dms_recapp_4_tail.txt\", \"w\")\n\trecapp4file.write(stdout)\n\trecapp4file.close\nend","stapler-class":"hudson.plugins.ruby.Ruby"}

,"kind":"org.jenkinsci.plugins.postbuildscript.PostBuildScript","scriptOnlyIfSuccess":false,"stapler-class":"org.jenkinsci.plugins.postbuildscript.PostBuildScript"}
Stacktrace:

javax.servlet.ServletException: java.lang.RuntimeException: Failed to instantiate class org.jenkinsci.plugins.postbuildscript.PostBuildScript from {"buildStep":

{"kind":"hudson.plugins.ruby.Ruby","ruby":"require 'net/ssh'\n\nNet::SSH.start('aaa.bbb.ccc.ddd', '********', :password => \"********\") do |ssh|\n\tstdout = \"\"\n\tssh.exec!(\"tail -n100 /logs/xyz.log\") do |channel, stream, data|\n\t\tstdout << data if stream == :stdout\n\tend\n \n\trecapp3file = File.new(ENV['WORKSPACE']+\"/philips/results/dms_recapp_3_tail.txt\", \"w\")\n\trecapp3file.write(stdout)\n\trecapp3file.close\nend\n\nNet::SSH.start('aaa.bbb.ccc.ddd', '********', :password => \"********\") do |ssh|\n\tstdout = \"\"\n\tssh.exec!(\"tail -n100 /logs/xyz.log\") do |channel, stream, data|\n\t\tstdout << data if stream == :stdout\n\tend\n \n\trecapp4file = File.new(ENV['WORKSPACE']+\"/philips/results/dms_recapp_4_tail.txt\", \"w\")\n\trecapp4file.write(stdout)\n\trecapp4file.close\nend","stapler-class":"hudson.plugins.ruby.Ruby"}

,"kind":"org.jenkinsci.plugins.postbuildscript.PostBuildScript","scriptOnlyIfSuccess":false,"stapler-class":"org.jenkinsci.plugins.postbuildscript.PostBuildScript"}
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:615)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:658)
at org.kohsuke.stapler.MetaClass$6.doDispatch(MetaClass.java:241)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:573)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:658)
at org.kohsuke.stapler.MetaClass$6.doDispatch(MetaClass.java:241)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:573)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:658)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:487)
at org.kohsuke.stapler.Stapler.service(Stapler.java:164)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:45)
at winstone.ServletConfiguration.execute(ServletConfiguration.java:248)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:333)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:376)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:95)
at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:206)
at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:179)
at net.bull.javamelody.PluginMonitoringFilter.doFilter(PluginMonitoringFilter.java:86)
at org.jvnet.hudson.plugins.monitoring.HudsonMonitoringFilter.doFilter(HudsonMonitoringFilter.java:84)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:98)
at hudson.plugins.greenballs.GreenBallFilter.doFilter(GreenBallFilter.java:58)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:98)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:87)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:194)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:366)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:194)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:366)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:166)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:173)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.ApiTokenFilter.doFilter(ApiTokenFilter.java:63)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:66)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:194)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:366)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:50)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:194)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:366)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:194)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:366)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:331)
at winstone.RequestHandlerThread.processRequest(RequestHandlerThread.java:215)
at winstone.RequestHandlerThread.run(RequestHandlerThread.java:138)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.RuntimeException: Failed to instantiate class org.jenkinsci.plugins.postbuildscript.PostBuildScript from {"buildStep":

{"kind":"hudson.plugins.ruby.Ruby","ruby":"require 'net/ssh'\n\nNet::SSH.start('aaa.bbb.ccc.ddd', '********', :password => \"********\") do |ssh|\n\tstdout = \"\"\n\tssh.exec!(\"tail -n100 /logs/xyz.log\") do |channel, stream, data|\n\t\tstdout << data if stream == :stdout\n\tend\n \n\trecapp3file = File.new(ENV['WORKSPACE']+\"/philips/results/dms_recapp_3_tail.txt\", \"w\")\n\trecapp3file.write(stdout)\n\trecapp3file.close\nend\n\nNet::SSH.start('aaa.bbb.ccc.ddd', '********', :password => \"********\") do |ssh|\n\tstdout = \"\"\n\tssh.exec!(\"tail -n100 /logs/xyz.log\") do |channel, stream, data|\n\t\tstdout << data if stream == :stdout\n\tend\n \n\trecapp4file = File.new(ENV['WORKSPACE']+\"/philips/results/dms_recapp_4_tail.txt\", \"w\")\n\trecapp4file.write(stdout)\n\trecapp4file.close\nend","stapler-class":"hudson.plugins.ruby.Ruby"}

,"kind":"org.jenkinsci.plugins.postbuildscript.PostBuildScript","scriptOnlyIfSuccess":false,"stapler-class":"org.jenkinsci.plugins.postbuildscript.PostBuildScript"}
at hudson.model.Descriptor.newInstance(Descriptor.java:602)
at hudson.model.Descriptor.newInstancesFromHeteroList(Descriptor.java:939)
at hudson.model.Descriptor.newInstancesFromHeteroList(Descriptor.java:926)
at hudson.util.DescribableList.rebuildHetero(DescribableList.java:203)
at hudson.model.Project.submit(Project.java:201)
at hudson.model.Job.doConfigSubmit(Job.java:1045)
at hudson.model.AbstractProject.doConfigSubmit(AbstractProject.java:722)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:288)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:151)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:90)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:111)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:573)
... 67 more
Caused by: java.lang.IllegalArgumentException: Failed to instantiate class org.jenkinsci.plugins.postbuildscript.PostBuildScript from {"buildStep":

{"kind":"hudson.plugins.ruby.Ruby","ruby":"require 'net/ssh'\n\nNet::SSH.start('aaa.bbb.ccc.ddd', '********', :password => \"********\") do |ssh|\n\tstdout = \"\"\n\tssh.exec!(\"tail -n100 /logs/xyz.log\") do |channel, stream, data|\n\t\tstdout << data if stream == :stdout\n\tend\n \n\trecapp3file = File.new(ENV['WORKSPACE']+\"/philips/results/dms_recapp_3_tail.txt\", \"w\")\n\trecapp3file.write(stdout)\n\trecapp3file.close\nend\n\nNet::SSH.start('aaa.bbb.ccc.ddd', '********', :password => \"********\") do |ssh|\n\tstdout = \"\"\n\tssh.exec!(\"tail -n100 /logs/xyz.log\") do |channel, stream, data|\n\t\tstdout << data if stream == :stdout\n\tend\n \n\trecapp4file = File.new(ENV['WORKSPACE']+\"/philips/results/dms_recapp_4_tail.txt\", \"w\")\n\trecapp4file.write(stdout)\n\trecapp4file.close\nend","stapler-class":"hudson.plugins.ruby.Ruby"}

,"kind":"org.jenkinsci.plugins.postbuildscript.PostBuildScript","scriptOnlyIfSuccess":false,"stapler-class":"org.jenkinsci.plugins.postbuildscript.PostBuildScript"}
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:582)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:377)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:373)
at hudson.model.Descriptor.newInstance(Descriptor.java:593)
... 83 more
Caused by: java.lang.IllegalArgumentException: Failed to convert the buildStep parameter of the constructor public org.jenkinsci.plugins.postbuildscript.PostBuildScript(java.util.List,java.util.List,java.util.List,boolean,java.util.List)
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:576)
... 86 more
Caused by: java.lang.IllegalArgumentException: Failed to instantiate interface hudson.tasks.BuildStep from

{"kind":"hudson.plugins.ruby.Ruby","ruby":"require 'net/ssh'\n\nNet::SSH.start('aaa.bbb.ccc.ddd', '********', :password => \"********\") do |ssh|\n\tstdout = \"\"\n\tssh.exec!(\"tail -n100 /logs/xyz.log\") do |channel, stream, data|\n\t\tstdout << data if stream == :stdout\n\tend\n \n\trecapp3file = File.new(ENV['WORKSPACE']+\"/philips/results/dms_recapp_3_tail.txt\", \"w\")\n\trecapp3file.write(stdout)\n\trecapp3file.close\nend\n\nNet::SSH.start('aaa.bbb.ccc.ddd', '********', :password => \"********\") do |ssh|\n\tstdout = \"\"\n\tssh.exec!(\"tail -n100 /logs/xyz.log\") do |channel, stream, data|\n\t\tstdout << data if stream == :stdout\n\tend\n \n\trecapp4file = File.new(ENV['WORKSPACE']+\"/philips/results/dms_recapp_4_tail.txt\", \"w\")\n\trecapp4file.write(stdout)\n\trecapp4file.close\nend","stapler-class":"hudson.plugins.ruby.Ruby"}

at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:582)
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:618)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:377)
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:574)
... 86 more
Caused by: org.kohsuke.stapler.NoStaplerConstructorException: There's no @DataBoundConstructor on any constructor of class hudson.plugins.ruby.Ruby
at org.kohsuke.stapler.ClassDescriptor.loadConstructorParamNames(ClassDescriptor.java:176)
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:561)
... 89 more