Note that the stapler-class entry is posted as an array, which is wrong. Maybe the libvirt-slave causes some layout issue?

The reporter also mentioned that he uninstalled the libvirt package from OS. Possibly related.

I have a correctly-working libvirt cloud, and the libvirt plugin. Same issue.
So it's maybe really related to the plugin?

I can duplicate this issue with jenkins 1.426, with libvirt-slaves 1.6 plugin.

java.lang.RuntimeException: Failed to instantiate class hudson.slaves.DumbSlave from {"":["hudson.plugins.sshslaves.SSHLauncher","hudson.slaves.RetentionStrategy$Always"],"labelString":"","launcher":

,"mode":"NORMAL","name":"dpb-l-g-i386","nodeDescription":"","nodeProperties":{"hudson-slaves-EnvironmentVariablesNodeProperty":{"env":[

,

]},"stapler-class-bag":"true"},"numExecutors":"1","remoteFS":"/home/jenkins","retentionStrategy":{"stapler-class":"hudson.slaves.RetentionStrategy$Always"}}
at hudson.model.Descriptor.newInstance(Descriptor.java:560)
at hudson.model.Node.reconfigure(Node.java:410)
at hudson.model.Computer.doConfigSubmit(Computer.java:1050)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:616)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:282)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:149)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:88)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:104)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:561)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:646)
at org.kohsuke.stapler.MetaClass$12.dispatch(MetaClass.java:374)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:561)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:646)
at org.kohsuke.stapler.MetaClass$4.doDispatch(MetaClass.java:196)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:561)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:646)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:477)
at org.kohsuke.stapler.Stapler.service(Stapler.java:159)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:45)
at winstone.ServletConfiguration.execute(ServletConfiguration.java:249)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:335)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:378)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
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:61)
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$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:333)
at winstone.RequestHandlerThread.processRequest(RequestHandlerThread.java:244)
at winstone.RequestHandlerThread.run(RequestHandlerThread.java:150)
at java.lang.Thread.run(Thread.java:679)
Caused by: java.lang.IllegalArgumentException: Failed to instantiate class hudson.slaves.DumbSlave from {"":["hudson.plugins.sshslaves.SSHLauncher","hudson.slaves.RetentionStrategy$Always"],"labelString":"","launcher":

,"mode":"NORMAL","name":"dpb-l-g-i386","nodeDescription":"","nodeProperties":{"hudson-slaves-EnvironmentVariablesNodeProperty":{"env":[

,

]},"stapler-class-bag":"true"},"numExecutors":"1","remoteFS":"/home/jenkins","retentionStrategy":{"stapler-class":"hudson.slaves.RetentionStrategy$Always"}}
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:628)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:372)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:368)
at hudson.model.Descriptor.newInstance(Descriptor.java:551)
... 62 more
Caused by: java.lang.IllegalArgumentException: Failed to convert the launcher parameter of the constructor public hudson.slaves.DumbSlave(java.lang.String,java.lang.String,java.lang.String,java.lang.String,hudson.model.Node$Mode,java.lang.String,hudson.slaves.ComputerLauncher,hudson.slaves.RetentionStrategy,java.util.List) throws java.io.IOException,hudson.model.Descriptor$FormException
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:622)
... 65 more
Caused by: java.lang.IllegalArgumentException: Failed to instantiate class hudson.slaves.ComputerLauncher from

at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:628)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:372)
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:620)
... 65 more
Caused by: java.lang.IllegalArgumentException: Class ["hudson.plugins.sshslaves.SSHLauncher","hudson.slaves.JNLPLauncher","hudson.slaves.CommandLauncher","hudson.os.windows.ManagedWindowsServiceLauncher","hudson.plugins.libvirt.VirtualMachineLauncher"] is specified in JSON, but no such class found in classLoader hudson.PluginManager$UberClassLoader
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:601)
... 67 more
Caused by: java.lang.ClassNotFoundException: ["hudson.plugins.sshslaves.SSHLauncher","hudson.slaves.JNLPLauncher","hudson.slaves.CommandLauncher","hudson.os.windows.ManagedWindowsServiceLauncher","hudson.plugins.libvirt.VirtualMachineLauncher"]
at hudson.PluginManager$UberClassLoader.findClass(PluginManager.java:656)
at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:596)
... 67 more

the .hpi file with the fix applied

Had the same problem... I managed to fix this... at least on my machines.
What I found was that the DumbSlave page was affected because the VirtualMachineLauncher's Descriptor was added to the list of getComputerLauncherDescriptors() and somehow one of the functions in configure-entries.jelly was not supported for that Descriptor. It seems that the getConfigPage did the trick.

<f:dropdownList name="slave.launcher" title="${%Launch method}"
help="${descriptor.getHelpFile('launcher')}">
<j:forEach var="d" items="${h.getComputerLauncherDescriptors()}">
<f:dropdownListBlock value="${d.clazz.name}" name="${d.displayName}"
selected="${it.launcher.descriptor==d}"
title="${d.displayName}">
<j:set var="descriptor" value="${d}"/>
<j:set var="instance"
value="${it.launcher.descriptor==d ? it.launcher : null}"/>
<f:invisibleEntry>
<input type="hidden" name="stapler-class" value="${d.clazz.name}" />
</f:invisibleEntry>
<st:include from="${d}" page="${d.configPage}" optional="true"/>
</f:dropdownListBlock>
</j:forEach>
</f:dropdownList>

I've attached the .hpi file give it a try

Sorry, I've attached twice .
Oh, by the way, it fixes JENKINS-8240 and JENKINS-7179

Sadly, although this bug is fixed we decided not to use this plugin because it does not support snapshot revert.

Have a nice day

Code changed in jenkins
User: tastybug
Path:
src/main/java/hudson/plugins/libvirt/Hypervisor.java
src/main/java/hudson/plugins/libvirt/PluginImpl.java
src/main/java/hudson/plugins/libvirt/VirtualMachineLauncher.java
src/main/java/hudson/plugins/libvirt/VirtualMachineSlave.java
src/main/resources/hudson/plugins/libvirt/VirtualMachineSlave/configure-entries.jelly
src/main/resources/index.jelly
src/main/webapp/help-libvirt-computerName.html
src/main/webapp/help-libvirt-hypervisorHost.html
src/main/webapp/help-libvirt-hypervisorSshPort.html
src/main/webapp/help-libvirt-hypervisorType.html
src/main/webapp/help-libvirt-hypervisorUri.html
http://jenkins-ci.org/commit/libvirt-slave-plugin/550e3286fa7df9514498562d4ba28ddf5784c1c1
Log:
-Fixed JENKINS-14468
-Fixed JENKINS-9471 (and duplicates JENKINS-8240, JENKINS-7179)
-Some comments improved

Fixed in 1.7

closing as fixed