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

For me the following commands followed by a reboot of the guest system helped:

cd /opt/VBoxGuestAdditions-*/init  
sudo ./vboxadd setup

I found the solution here. Referring to this post you need to stop vboxadd before triggering the commands (I didn't do so since I don't know what vboxadd is).