Comment 2 for bug 1572001

it turned out virtual-box-guest-dkms was not installed at all (I formerly did apt install virtualbox-guest-x11 to install the guest utilities; are there any dependencies missing?)

after

apt install virtual-box-guest-dkms

I got an error message (in the terminal):

module 5.0.16_ubuntu for vboxvideo.ko is not newer than what is already found in kernel 4.4.0-18-generic. You may override by specifiying --force.

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I now tried

dpkg-reconfigure virtualbox-guest-dkms --force

same error again; I don't know for which command I should use --force

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I finally tried:

dkms build -m virtualbox-guest -v 5.0.16
Module virtualbox-guest/5.0.16 already built for kernel 4.4.0-18-generic/4

dkms install -m virtualbox-guest -v 5.0.16
Module virtualbox-guest/5.0.16 already installed on kernel 4.4.0-18-generic/x86_64

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after a reboot, the problem persists (as expected)