Fix failing `vagrant up` error

$ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...  
==> default: Clearing any previously set forwarded ports...
There was an error while executing `VBoxManage`, a CLI used by Vagrant  
for controlling VirtualBox. The command and stderr is shown below.

Command: ["modifyvm", "76589a4c-eee9-413f-df45-ff485aef8e44", "--natpf1", "delete", "ssh", "--natpf1", "delete", "tcp27017", "--natpf1", "delete", "tcp3000", "--natpf1", "delete", "tcp35529"]

Stderr: VBoxManage: error: An unexpected process (PID=0x0000B059) has tried to lock the machine 'web_default_1440666168158_63453', while only the process started by LaunchVMProcess (PID=0x0000AF9A) is allowed  
VBoxManage: error: Details: code E_ACCESSDENIED (0x80070005), component MachineWrap, interface IMachine, callee nsISupports  
VBoxManage: error: Context: "LockMachine(a->session, LockType_Write)" at line 492 of file VBoxManageModifyVM.cpp  

To fix this, restart VirtualBox.app through the command line. Be careful to use the correct user. By using sudo here, you could potentially open a VirtualBox instance that is owned by the sudo user.

/Applications/VirtualBox.app/Contents/MacOS/VirtualBox restart

Now vagrant up will work as expected.