First of all, I know there has been a lot of threads about Ubuntu login loops. I have read them and I feel like I've tried everything they proposed but nothing works.

Background

I have VirtualBox installed with an Ubuntu 14.04 64 bits VM. I've had it for a while now, using it almost everyday. I've never had any problem with it. Two days ago, I installed a VirtualBox update and the latest Guest Additions. After installing the Guest Additions, I found that I couldn't mount my shared folder anymore, so I restarted the VM, tried again, and it still didn't work. I reinstalled the Guest Additions, restarted the VM, and then I could mount the shared folder just fine. However, I had lost the copy and paste function between my host (Windows 10) and my guest (Ubuntu). At the end of the day, I turned off the VM. The next day, I was stuck in a login loop (with my usual session and with the guest session). Before going back to the login page, I have a black screen that says:

* Starting LightDM Display Manager [OK]
* Stopping Send an event to indicate plymouth is up [OK]

What I tried

Checking virtualization

First, I checked the .xsession-errors file which indicated something about virtualization maybe being disabled and something about at-spi2-core. I didn't think of making a copy of this file and I deleted it since. I tried restarting several times since then and it stays empty now. Anyway, I checked that 3D Acceleration was still activated in the VM setting (it was) and that virtualization was still activated in the BIOS (it was).

Checking Guest Additions

I then tried reinstalling the Guest Additions using the following code :

# Insert Guest Additions
mkdir /tmp/cd
sudo mount /dev/sr0 /tmp/cd
sudo /tmp/cd/VBoxLinuxAdditions.run

The installation went normal, but it didn't help the login loop.

Checking permissions

Following some threads about login loops, I checked permissions in /tmp ; /home/myusername and /home/.Xauthority. Everything was fine.

Checking disk space

I also checked that my disk space wasn't full (even though it wouldn't have made sense since autocompletion worked) using df -h ; I still had 15 Gb of free space.

Checking updates

I also tried updating using sudo apt-get update && sudo apt-get dist-upgrade but there was nothing to update.

Checking lightdm

I also tried some code found on other threads regarding login loops and lightdm :

sudo dpkg-reconfigure lightdm
sudo service lightdm restart
sudo reboot

It didn't help.

Checking random stuff found on login loops threads cause I really need my VM to work

Didn't help:

sudo apt-get install --reinstall at-spi2-core
sudo apt-get install --reinstall unity
sudo apt-get install --reinstall ubuntu-desktop

Didn't work (but not sure it should work since I'm on a VM):

sudo apt-get purge --auto-remove fglrx*
sudo dpkg-reconfigure xserver-xorg
sudo apt-get install fglrx # didn't work

Checked that I didn't have Xenial or other packages mixed with Trusty packages (using apt-cache): I don't, they all seem to be Trusty packages.

I also tried adding a new user with sudo priviledges but I also can't login to the new user, I'm still in a login loop.

EDIT: Additional information

I don't know if this is relevant at all, but I made a new VM with a fresh install of Ubuntu 14.04 64 bits. I installed the Guest Additions, and each time I log into the VM, I have the following error message: VBoxClient: VBoxClient (seamless): failed to start. Stage: Setting guest IRQ filter mask Error: VERR_INTERNAL_ERROR.
The only things I did on this VM are installing updates & Guest Additions. I enabled the copy/paste function between host and guest but it doesn't work either.

My question

I ran out of options to help my login loop. I feel like I've tried everything I read on my Google searches. Does anyone have any input or idea about what I could try to make it work ?