I have the plugin configured to suspend VMs when jobs finished.
I noticed that sometimes starting a VM slave in Parallels Desktop takes way too long for Jenkins, so the master stops attempts to connect to the slave and takes it offline with 'This node is offline because Jenkins failed to launch the slave agent on it' message. In the slave log there's

[12/27/16 14:36:50] [SSH] Opening SSH connection to VMHostname:22.
Operation timed out
ERROR: Unexpected error in launching a slave. This is probably a bug in Jenkins.
java.lang.IllegalStateException: Connection is not established!

I can launch the slave manually, and then everything is fine: the waiting jobs run there, the slave is suspended and removed from Jenkins.

Looks like this happens because the timeout for VM startup is set to 60 seconds in ParallelsDesktopConnectorSlaveComputer.java:getVmIPAddress(). Probably, it should have a bigger timeout and check for VM status in this loop.