First, download and run the VirtualBox_Uninstall.tool (mirror).

Then, run

sudo kextcache -invalidate /

Note: the kextcache command must be run as root.

You should see something like KernelCache ID: <Some ID>.

Finally, make sure there is no folder named VirtualBox in /Library/StagedExtensions any more. Run the following and you shouldn’t see any output:

find /Library/StagedExtensions -name VirtualBox

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