First bad symptom on Monday was some fonts disappearing (!!) when windows were given focus after being in background -- happening in Eclipse editors, Chrome pages and some toolbars too. The problem repeated a while after rebooting. So I decided to go for apt-get dist-upgrade, as maybe the most recent update would help.
That was just where problems really started. Conflicts, and packages unconfigured. Booting led to the graphical loading screen, without ever reaching a login, and no network connection either!! (Total panic at this point).
Doing some apt-get -f install , and dpkg --configure -a, did help finishing to install some packages. There was one big problem left:
(Reading database ... 441700 files and directories currently installed.)
Unpacking ubuntu-release-upgrader-core (from ubuntu-release-upgrader-core_1%3a0.190.8_all.deb) ...
dpkg: error processing ubuntu-release-upgrader-core_1%3a0.190.8_all.deb (--install):
trying to overwrite '/etc/update-manager/release-upgrades', which is also in package update-manager-core 1:0.174.4
I have no idea where this came from. I was unable to solve it cleanly. The last hack was to install the .deb manually (dpkg -i), with a --force flag to ignore the overwritten file! This IS WRONG, but allowed me to continue and finish upgrading the rest of the packages left behind.
With packages finally updated, the machine still did not show a login screen. Networking did work, on consoles. No errors anywhere. I believe there was no greeter configured... so I ran dpkg-reconfigure lightdm.
After some reconfiguration, and choosing lightdm as greeter, I was able to log back in to the desktop. I lost my old greeter (I believe I was running gdm) but I cannot even boot now. I will have to continue debugging later... this week this machine just needs to work. (Which raises the question: Why on earth did I start all the updates this last weekend?).