On my Macbook Pro, I run a virtual Ubuntu machine thanks to VMware Fusion. This works great and I am very happy with it. However, when I upgraded it from 12.04 LTS to 14.04 LTS, I could no longer login via the graphical login screen.
My username had disappeared from there – only the Guest session was available. Logging in via ssh or console (Ctrl + Alt + F1) worked just fine.
It turned out that my user’s uid (user id) was causing this. Right after installing Ubuntu, I had changed the uid of my Ubuntu user to the same as on the Mac (501), to make file sharing easier between my Mac and the Ubuntu VM. It looks like the login/session manager of Ubuntu 14.04 wants a uid of at least 1000. Logical, because Ubuntu considers uids of smaller than 1000 to be system accounts.
In /etc/lightdm/users.conf I found the following line:
minimum-uid=500
However, a comment in that same file suggested that changing this would not help:
# NOTE: If you have AccountsService installed on your system, then LightDM will # use this instead and these settings will be ignored
From someone else’s question about AccountsService and LightDM, I learned that the file login.defs holds this information. After changing the line
UID_MIN 1000
to
UID_MIN 500
and rebooting, I could login again. Problem fixed.
An alternative solution is to change the uid on the Macbook, as described in this post. I was a bit more afraid to break things on the Macbook than on the Ubuntu VM, which is why I chose the above solution.