Unable to install VirtualBox 5.1 on Ubuntu 16.04

[devray]

I am trying to install VirtualBox 5.1 (5.1.18) on my Ubuntu 16.04 desktop.
There appears to be no errors in vbox-install.log, nor dmesg. (Zipped files attached)

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$ uname -a
Linux mozart 4.8.0-41-generic #44~16.04.1-Ubuntu SMP Fri Mar 3 17:11:16 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

The install fails as follows:

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$ sudo apt-get install virtualbox-5.1
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following NEW packages will be installed:
  virtualbox-5.1
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/65.4 MB of archives.
After this operation, 159 MB of additional disk space will be used.
Preconfiguring packages ...
Selecting previously unselected package virtualbox-5.1.
(Reading database ... 190095 files and directories currently installed.)
Preparing to unpack .../virtualbox-5.1_5.1.18-114002~Ubuntu~xenial_amd64.deb ...
Unpacking virtualbox-5.1 (5.1.18-114002~Ubuntu~xenial) ...
Processing triggers for systemd (229-4ubuntu16) ...
Processing triggers for ureadahead (0.100.0-19) ...
Processing triggers for hicolor-icon-theme (0.15-0ubuntu1) ...
Processing triggers for shared-mime-info (1.5-2ubuntu0.1) ...
Processing triggers for desktop-file-utils (0.22-1ubuntu5.1) ...
Processing triggers for gnome-menus (3.13.3-6ubuntu3.1) ...
Processing triggers for bamfdaemon (0.5.3~bzr0+16.04.20160824-0ubuntu1) ...
Rebuilding /usr/share/applications/bamf-2.index...
Processing triggers for mime-support (3.59ubuntu1) ...
Setting up virtualbox-5.1 (5.1.18-114002~Ubuntu~xenial) ...
addgroup: The group `vboxusers' already exists as a system group. Exiting.
vboxdrv.sh: failed: modprobe vboxdrv failed. Please use 'dmesg' to find out why.

There were problems setting up VirtualBox.  To re-start the set-up process, run
  /sbin/vboxconfig
as root.

(I also tried installing virtualbox 5.1.14 and had the same failure.)
Please advise.

Thanks.

[Umely]

UEFI Secure Boot Enable?

[socratis]

On top of Umely's question, I have another issue:

$ sudo apt-get install virtualbox-5.1

This will get you the Ubuntu's forked VirtualBox version from the Ubuntu repositories. Please uninstall/clean/purge that one and install the official one from: https://www.virtualbox.org/wiki/Downloads

[devray]

Yes, UEFI Secure Boot was enabled. Once I disabled that secure boot I was able to install VirtualBox 5.1 (which is the official version v5.1.18 -- not the Ubuntu fork. I believe $ sudo apt-get install virtualbox installs v5.0.32, the ubuntu fork)

Thanks!