I met a problem, the plugin failed to add any of my VirtualBox machine nodes.
I start the commands successfully in the virtualbox host:

VBoxManage setproperty websrvauthlibrary null
vboxwebsrv

and I have connected to VirtualBox host successfully in Jenkins("Manage Jenkins" page)
But still can't add node, the output slave log on Jenkins as follow:

[VirtualBox] Sleep before stage 2 launcher, attempt 1
[VirtualBox] Starting stage 2 launcher (JNLPLauncher)
[VirtualBox] Stage 2 launcher completed
[VirtualBox] Sleep before stage 2 launcher, attempt 2
[VirtualBox] Starting stage 2 launcher (JNLPLauncher)
[VirtualBox] Stage 2 launcher completed
[VirtualBox] Sleep before stage 2 launcher, attempt 3
[VirtualBox] Starting stage 2 launcher (JNLPLauncher)
[VirtualBox] Stage 2 launcher completed
[VirtualBox] Sleep before stage 2 launcher, attempt 4
[VirtualBox] Starting stage 2 launcher (JNLPLauncher)
[VirtualBox] Stage 2 launcher completed
[VirtualBox] Sleep before stage 2 launcher, attempt 5
[VirtualBox] Starting stage 2 launcher (JNLPLauncher)
[VirtualBox] Stage 2 launcher completed

Jenkins try to connect to Virtual Machine continuously
I'm not sure what the problem is...

The attached screen capture is Jenkins settings for VirtualBox