Plasma 5 doesn't start after update on Stretch

[Francesco De Vita <fradev@inventati.org>]

Hello list!

I'm on Debian Stretch and today, after an update, Plasma 5 doesn't
start anymore.

These are the packages updated this morning:

> $ cat /var/log/apt/history.log
> ...
> Start-Date: 2015-08-19  09:55:01
> Upgrade: libijs-0.35:amd64 (0.35-10, 0.35-11), colord-data:amd64
> (1.2.1-1, 1.2.11-1), libkf5itemviews-data:amd64 (5.12.0-1,
> 5.13.0-1), libkf5networkmanagerqt6:amd64 (5.12.0-1, 5.13.0-1),
> libkf5configgui5:amd64 (5.12.0-1, 5.13.0-1), libcolorhug2:amd64
> (1.2.1-1+b2, 1.2.11-1), libyaml-tiny-perl:amd64 (1.67-1, 1.69-1),
> libkdecorations2private5:amd64 (5.3.2-1, 5.3.2-2),
> libkf5config-data:amd64 (5.12.0-1, 5.13.0-1),
> libkdecorations2-5:amd64 (5.3.2-1, 5.3.2-2), libkf5configcore5:amd64
> (5.12.0-1, 5.13.0-1), libcolord2:amd64 (1.2.1-1+b2, 1.2.11-1),
> libkf5itemviews5:amd64 (5.12.0-1, 5.13.0-1), colord:amd64
> (1.2.1-1+b2, 1.2.11-1), libkf5modemmanagerqt6:amd64 (5.12.0-1,
> 5.13.0-1), libkf5guiaddons5:amd64 (5.12.0-1, 5.13.0-1)
> End-Date: 2015-08-19  09:55:09
> ...

First of all, sddm doesn't show at each reboot and when
it doesn't I have to restart it (# service sddm restart), it is the
first time I see it behaving like this. When I successfully log in I
see the splash screen of Plasma 5 but the progress bar gets stuck near
the end and then nothing happens.

I thought it could be a problem with some configuration files so I
created a new user in order to have a clean home dir and I tried to
login with it, but unfortunately the problem reappeared. Also I tested
the login using lightdm instead of sddm, but nothing changed. 

These tests lead me to think that this issue is related to the packages 
updated today. Do you have any suggestions to further investigate this 
problem? Is there someone else who experienced the same issue?

Please add me in CC if you reply, thank you.

Best regards
Francesco