I have experienced this bug as well. The only way I have been able to fix this is by deleting my ~/.config folder.
A better option is to simply move your .config folder into another folder
mv -f ~/.config ~/.backupconfig
in case you are unfamiliar with terminal commands:
'mv' is the move command and '-f' forces the move
This will return Unity to normal and allow you to use it again, though you will have to set up your Launcher panel again. I have yet to find a way to use both together, unfortunately, because I really like Lubuntu and Unity. I hope problems will be resolved when LXQt is finished and Unity migrates to Qt. I think the problems arise mainly from openbox, and some sort of conflicting config there.
It is possible, to install KDE run sudo apt-get install --no-install-recommends kubuntu-desktop.
After you install it you can switch between Desktop Environments at the login screen.
This also works with most of the common DE's out there.
For Xubuntu (XFCE) sudo apt-get install --no-install-recommends xubuntu-desktop
For Lubuntu (LXDE) sudo apt-get install --no-install-recommends lubuntu-desktop
And Gnome Shell (Gnome 3) sudo apt-get install gnome-core
Theoretically you can have as many of these installed as you want, but I would only use one or two at time to avoid conflicts.
Note: If you plan on moving to a new DE completely, remove the --no-install-recommends from the commands above.
Removing
You can remove any of these with the purge command in apt, like this:
sudo apt-get purge gnome-core
or
sudo apt-get purge kubuntu-desktop
If you want to try Linux Mint's Cinnamon use these commands:
sudo add-apt-repository ppa:gwendal-lebihan-dev/cinnamon-stable
sudo apt-get update
sudo apt-get install cinnamon
To uninstall Cinnamon use sudo apt-get purge cinnamon and follow these instructions on removing PPA's.
I have personally run Unity and Kubuntu, Unity and Xubuntu and Unity and Gnome Shell. I have used Cinnamon a little bit too. KDE is by far the most customizable followed by XFCE in Xubuntu. Right now I just use Unity.
Best Answer
Assuming you installed using the PPA referenced here, you can remove Cinnamon along with its dependencies using the following commands:
To remove Xubuntu-Desktop and its associated packages:
To remove Lubuntu-Desktop and its associated packages:
To remove Gnome-Shell/Gnome-Fallback and its associated packages:
Clean up any now-unnecessary .deb packages and libraries:
Ensure that all package dependencies are still intact (while upgrading your existing packages):