Upgrading VirtualBox from the former version 3.2 to 4.0.2 broke the virtualbox-plugin. To be more precise, the plugin can no longer connect to the virtualbox-cloud using the http server (vboxwebsrv).

The virtualbox-webserver is started with:
1) VBoxManage setproperty websrvauthlibrary null
2) vboxwebsrv

When testing the connection to the cloud from inside Hudson system-settings, the following error messages is printed:
"HTTP transport error: java.net.ConnectException: Connection refused: connect"

And the main Hudson log shows no exception dump or whatsoever, only the INFO message:
"INFO: Testing connection to http://csgwfcl185:18083/virtualbox with username CISystem"

Instead when trying to startup a slave-node explictly, the following exception-dump is printed:

FATAL: HTTP transport error: java.net.ConnectException: Connection refused: connect
com.sun.xml.internal.ws.client.ClientTransportException: HTTP transport error: java.net.ConnectException: Connection refused: connect
at com.sun.xml.internal.ws.transport.http.client.HttpClientTransport.getOutput(HttpClientTransport.java:121)
at com.sun.xml.internal.ws.transport.http.client.HttpTransportPipe.process(HttpTransportPipe.java:142)
at com.sun.xml.internal.ws.transport.http.client.HttpTransportPipe.processRequest(HttpTransportPipe.java:83)
at com.sun.xml.internal.ws.transport.DeferredTransportPipe.processRequest(DeferredTransportPipe.java:105)
at com.sun.xml.internal.ws.api.pipe.Fiber.__doRun(Fiber.java:587)
at com.sun.xml.internal.ws.api.pipe.Fiber._doRun(Fiber.java:546)
at com.sun.xml.internal.ws.api.pipe.Fiber.doRun(Fiber.java:531)
at com.sun.xml.internal.ws.api.pipe.Fiber.runSync(Fiber.java:428)
at com.sun.xml.internal.ws.client.Stub.process(Stub.java:211)
at com.sun.xml.internal.ws.client.sei.SEIStub.doProcess(SEIStub.java:124)
at com.sun.xml.internal.ws.client.sei.SyncMethodHandler.invoke(SyncMethodHandler.java:98)
at com.sun.xml.internal.ws.client.sei.SyncMethodHandler.invoke(SyncMethodHandler.java:78)
at com.sun.xml.internal.ws.client.sei.SEIStub.invoke(SEIStub.java:107)
at $Proxy47.iWebsessionManagerLogon(Unknown Source)
at com.sun.xml.ws.commons.virtualbox_3_1.IWebsessionManager.logon(IWebsessionManager.java:255)
at hudson.plugins.virtualbox.VirtualBoxUtils.connect(VirtualBoxUtils.java:29)
at hudson.plugins.virtualbox.VirtualBoxUtils.getMachines(VirtualBoxUtils.java:39)
at hudson.plugins.virtualbox.VirtualBoxCloud.retrieveMachines(VirtualBoxCloud.java:56)
at hudson.plugins.virtualbox.VirtualBoxCloud.getVirtualMachines(VirtualBoxCloud.java:61)
at hudson.plugins.virtualbox.VirtualBoxPlugin.getVirtualBoxMachine(VirtualBoxPlugin.java:94)
at hudson.plugins.virtualbox.VirtualBoxComputerLauncher.launch(VirtualBoxComputerLauncher.java:40)
at hudson.slaves.SlaveComputer$1.call(SlaveComputer.java:184)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
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.net.ConnectException: Connection refused: connect
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:333)
at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:195)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:182)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)
at java.net.Socket.connect(Socket.java:529)
at java.net.Socket.connect(Socket.java:478)
at sun.net.NetworkClient.doConnect(NetworkClient.java:163)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:394)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:529)
at sun.net.www.http.HttpClient.<init>(HttpClient.java:233)
at sun.net.www.http.HttpClient.New(HttpClient.java:306)
at sun.net.www.http.HttpClient.New(HttpClient.java:323)
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:975)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:954)
at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:841)
at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:1019)
at com.sun.xml.internal.ws.transport.http.client.HttpClientTransport.getOutput(HttpClientTransport.java:109)
... 26 more