Hi, please help me make my KDE desktop usable again.

System:
Kubuntu 14.04 on an AMD CPU/GPU. Faithfully applied every update that comes along.

Symptoms, started yesterday on login:
All autostart windows appear on the same desktop.
Taskbar with 4 desktops appears but does not react at all.
Cannot open any new windows - <Alt>-<F2> does nothing, KDE menu does not open.
X11 seems to work, e.g. clicking on <close> in a kalarm notifier window makes it go away.

What I tried:
a) Log into Gnome desktop - works OK, no problems.
b) Log into KDE as different user - same symptoms as described.
c) Apt-get install --reinstall kubuntu-desktop - no change.
d) Apt-get install --reinstall kde-workspace - no change.
e) Compare (cmp) all executables in /usr/bin with backup from 2 January - found nothing suspicious, differences seem to be due to updates. (Backup from 2 Feb. is corrupt - go figure.)
f) inspected /var/log/messages - nothing obvious.
g) inspected /var/log/syslog - nothing obvious.

Because of b) I don't think my config files are to blame, should be an executable.

What can I try next?

Thank you for any pointers.

What version of KDE are you using? Is it Plasma5? Do you use sddm? Some details about your KDE version would be useful.
jdk

I don know offhand the KDE and Plasma versions, they are what comes with Kubuntu 14.04 and faithfully installed updates. But read on since it may not matter, I now think I have a hardware problem, and I'm reduced to writing this on a RaspberryPi. This is what happened:

I managed to uncompress the backup, it wasn broken after all.
I tried restoring all of "/" from backup - that didn't go over too well on the running system.
I booted Knoppix from a DVD and did the restore there. (Screwy: My root drive is on a software (mdadm) RAID1 and Knoppix would not let me assemble both drives, but I forced it to run the array in degraded mode.)
I then booted from the hard disk again, and KDE was OK, so I let it rebuild the array. Big, premature sigh of relief.

After the next reboot KDE was broken again, as was the Gnome desktop. In KDE I got a popup telling me that /var/lib/dbus-1.0/dbus-daemon-launch-helper failed to run. That time, and ever since, there is no taskbar at all. Also, strangely, the options for Shutdown and Reboot have disappeared from both the KDE menu and from the pulldown on the login screen. Gnome doesn't even start anymore, it just cycles back to the login screen after I enter my passwword.
I did the Knoppix restore one more time, same result.
I ran memtest86: it found 1 bad address in my 16GB of DDR3.
I removed an 8GB module and ran memtest again - clean in ~2.5 passes through all the tests.
I did the Knoppix restore again - same dysfunctional KDE and Gnome. I'm dead in the water since there isn't even a terminal to do anything in.

So do you agree it looks like hardware?
BTW, the motherboard is a Gigabyte "Ultra-Durable", no kidding, less than a year old. The case and power supply are much older, could be 10 years. But the machine ran fine doing the memtest, and it pulled it from the hard drive (/boot is not on the RAID). The drives are several years old too, and I replaced one of them last year. But the RAID should protect me from read errors. Of course at the moment there is no redundancy since Knoppix only lets me restore to a single drive.

What next? Rebuild the array on the Raspberry? Would that even be compatible? Should I try replacing the power supply?

Thanks for any ideas