The following packages have unmet dependencies:
virtualbox-guest-x11 : Depends: xorg-video-abi-15
Depends: xserver-xorg-core (>= 2:1.14.99.902)
E: Unable to correct problems, you have held broken packages.
Running the installer for VirtualBox' guest additions (from VirtualBox guest additions ISO), throws this error:
Installing the Window System drivers
Warning: unknown version of the X Window System installed. Not installing X Window System drivers.
I'm suspecting that my Virtualbox version on the host is too old, so it doesn't know how to handle the X11 version of the 14.04 on the VM-client.
Therefore, I'm currently upgrading to Virtualbox v5 - from offical Oracle's repositories: https://www.virtualbox.org/wiki/Linux_Downloads
UPDATE:
SOLVED! That did the trick. Upgraded Virtualbox to v5 (on Xubuntu 12.04, precise), then re-ran the virtualbox-guest-additions "VBoxLinuxAdditions.run" as root.
Rebooted the VM-client (14.04), and now the screen resolution can be changed properly
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