VBoxManage: error: Failed to create the host-only adapter [closed]

I had the same problem today. The reason was that I had another VM running in VirtualBox.

Solution:

1. Open VirtualBox and shut down every VM running
2. Go to System Preferences > Security & Privacy Then hit the "Allow" button to let Oracle (VirtualBox) load.
3. Restart VirtualBox

sudo "/Library/Application Support/VirtualBox/LaunchDaemons/VirtualBoxStartup.sh" restart

You should now be able to run vagrant up or vagrant reload and have your new host configured.

As mentioned in this answer, recent versions of macOS can block VirtualBox.

Solution:

Go to System Preferences > Security & Privacy Then hit the "Allow" button to let Oracle (VirtualBox) load.

(thanks to @pazhyn, @lsimonetti & @dave-beauchesne for clarifications)

For Mac OS X 10.9.3 and Vagrant 1.6.3 and VirtualBox 4.3.6 this problem was fixed with restarting the VirtualBox

sudo "/Library/Application Support/VirtualBox/LaunchDaemons/VirtualBoxStartup.sh" restart

TL;DR MacOS is probably blocking VirtualBox. Go to System Preferences > Security & Privacy Then hit the "Allow".

Solution: Go to System Preferences > Security & Privacy Then hit the "Allow" button to let Oracle (VirtualBox) load.

MacOS by default can block kexts from loading. You must click the "allow" button before executing the VirtualBoxStartup.sh script.

Then run: sudo "/Library/Application Support/VirtualBox/LaunchDaemons/VirtualBoxStartup.sh" restart like the answer above.

(This article provides more clarity to MacOS kernel extension loading)

This issue appears to be fixed by installing the latest version of Virtual Box.