Can sign in to xfce but not to Ubuntu Desktop environment after update

it looks like you at some point tried to run your xsession as root and this has left some other files owned as root in your home dir.

to list all the files in your home dir owned by root

find ~ -uid 0

have a look at the list and chown anything back to you then try again.

It might also be a good idea to send this to a file for reference later in case you introduce more issues with later changes.

If you are happy that all the files listed by that command should be returned to your ownership then run this command.

find ~ -uid 0 -exec chown $USER:$USER -r {} \;

It would also be worth creating a fresh user account for comparison, try login in with that, if that doesn't work then the issue may not be at user level but could be a system thing.

1. Please check and post your /var/log/Xorg.0.log and /var/log/Xorg.0.log.old after the failed login.
2. Please remove x11-common, remove/rename /etc/X11 and install x11-common again.
3. XFCE does not use expensive 3D effects, so probably OpenGL and or special graphic drivers are not in place. Check graphic drivers for your card.