Vagrant was unable to mount VirtualBox shared folders

Question

Current setup: Virtualbox 5.1.20 with Vagrant 1.9.3

Previous working setup: Virtualbox 5.1.18 with Vagrant 1.9

Answer 1

Installing the vbguest plugin worked in my case with VirtualBox 5.2.6 and Vagrant 2.0.1:

vagrant plugin install vagrant-vbguest