Lubuntu Upgrade – Blank Desktop After Upgrading Lubuntu to the Next Version

Make sure you have enough free disk space.

After making sure permissions are correct on ~/.Xauthority and tmp as Scott Goodgame's answer explains, the next step is to make sure you have enough free disk space on the partition that contains /home (as Scott Goodgame also suggested in a comment).

You're able to log in to a Lubuntu Desktop session, so do so, then open an LXTerminal (Ctrl+Alt+T) and run:

df -h

That will show you each mounted partition and statistics about how much space is on it and how full it is. For example, when I run that on my machine, I get:

Filesystem          Size  Used Avail Use% Mounted on
/dev/sda8            15G  7.9G  6.2G  57% /
udev                997M   12K  997M   1% /dev
tmpfs               403M  900K  402M   1% /run
none                5.0M     0  5.0M   0% /run/lock
none               1007M  2.6M 1004M   1% /run/shm
none                100M   48K  100M   1% /run/user
/dev/sda6           240M   91M  138M  40% /boot
/dev/sda9            31G   29G  112M 100% /home

^{(I've left out some entries representing external hard drives and network drives, which would primarily serve to confuse matters.)}

If you have a separate /home partition listed in the Mounted on column, check if it has enough free space to spontaneously create files. Usually this doesn't have to be very much--20M is usually quite sufficient, but I recommend shooting for at least 50M. (And of course, you'd normally want much more free than that so you can download and create big files in your home directory--but as you can see, I only have a little over 100M, and I haven't experienced any problems.)

If you don't have a separate /home partition, then look at the free space on /.

Whether or not you have a separate /home partition, you can make it so your home directory (/home/username) has more free space by deleting files there. Make sure not to delete anything important though! Your Downloads folder is often a good place to check for files you don't need.

If you don't have a separate /home partition, then you can free space in your home directory by deleting files anywhere in / (so long as the location where you remove them is not inside a mount point for something else--for example, if a separate /boot partition listed, then you cannot free space in / by deleting files inside /boot even though /boot is a child directory if /).

Generally, it is best not to manually delete files in / (unless they're in your home directory and you do not need them). Instead, you can free space in / by uninstalling software. However, much software in Ubuntu depends on other software, so don't press y or click OK until you've read through the list of packages that would be removed alongside whatever you've specified for removal. You can remove packages in the Software Center, the Synaptic Package Manager, or on the command-line with apt-get.

If you do not have a separate /boot partition (I do, as you can see above, but these days most installations do not), then since everything in /boot would then be in /, you can free space in / by deleting files from /boot. Again, don't do this manually. Instead, uninstall old kernels:

• How do I remove old kernel versions to clean up the boot menu?

You need to uninstall them if you want to free space, not just hide them in the GRUB menu. (Answers there explain both.) Make sure you do not remove all your kernels, because then Ubuntu will not be able to boot, and the fix for that is not very much fun.

• It's a good idea to keep at least the kernel you are currently running (as told to you by uname -r).
• It's also advisable to keep one other kernel, both because the one you're currently using could have problems (which you could work around by falling back to another) and because then if you remove one more kernel than you intended, you still have one.
• If you do accidentally remove all kernels but have not rebooted, you can just install a kernel back with sudo apt-get install linux-image or sudo apt-get install linux-image-generic.

Once df -h shows that at least 50M of space is free on / and, if it is listed, on /home, try logging in to an Xubuntu session again.

Try a different display manager.

A display manager provides the graphical login screen. A problem logging in like the one you describe could be related to a problem with the display manager you are currently using. Unless you're using a very old version of Lubuntu, you're almost certainly using LightDM as your display manager. (You can check the picture in that Wikipedia article to see if it looks like your login screen.)

Since you have Lubuntu installed, LXDE's very own display manager is probably installed. This display manager is called LXDM (though the picture there will not necessarily look just like LXDM on your machine).

To make your system use LXDM instead of LightDM, run:

sudo dpkg-reconfigure lxdm

You should see:

When you see that, press Enter. (That clicks the OK "button.")

Then you'll be asked to choose between the display managers that are installed on your system. Most likely there will be two choices: lightdm and lxdm. Use the arrow keys to select lxdm and press Enter:

Then either reboot (properly from the power icon on the upper-right corner of the screen, or with sudo reboot) to apply the change. Or, if you prefer, apply the change in a virtual console (Ctrl+Alt+F1) by logging in and running:

sudo stop lightdm
sudo start lxdm

When you run that first command, all your graphical programs will be quit immediately, even if they have unsaved work. Any unsaved work will be lost. I recommend simply logging out before switching to the virtual console (though it's the running of the commands, and not switching to a virtual console, that will terminate your graphical applications).

On a virtual console, you can switch back to the GUI manually by pressing Alt+F7. But when you run the above commands, you should be placed back on the (new) graphical login screen automatically.

If you were unable to do any of this because dpkg-reconfigure failed and said lxdm was not installed, then just install the lxdm package. To install it from the command line, run:

sudo apt-get update && sudo apt-get install lxdm

If you install it in the command-line, it should run dpkg-reconfigure automatically. If it does not, you can still run it manually.

Hopefully, if making sure you have enough free space doesn't help, then switching to a different display manager as explained here will help. If it does not, please comment here and add as detailed information as possible to your question about what you did and what happened.