Seems to appear at random.
==> vb-fedora-24-np: Gracefully halting virtual machine...
==> vb-fedora-24-np: Executing custom VBoxManage commands...
vb-fedora-24-np: Executing: modifyvm vb-fedora-24-np --cableconnected1 on
==> vb-fedora-24-np: Error executing command: VBoxManage error: VBoxManage: error: The machine 'vb-fedora-24-np' is already locked for a session (or being unlocked)
==> vb-fedora-24-np: VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component MachineWrap, interface IMachine, callee nsISupports
==> vb-fedora-24-np: VBoxManage: error: Context: "LockMachine(a->session, LockType_Write)" at line 506 of file VBoxManageModifyVM.cpp
==> vb-fedora-24-np: Unregistering and deleting virtual machine...
==> vb-fedora-24-np: Deleting output directory...
Build 'vb-fedora-24-np' errored: Error executing command: VBoxManage error: VBoxManage: error: The machine 'vb-fedora-24-np' is already locked for a session (or being unlocked)
VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component MachineWrap, interface IMachine, callee nsISupports
VBoxManage: error: Context: "LockMachine(a->session, LockType_Write)" at line 506 of file VBoxManageModifyVM.cpp