kde login screen freezes at around 56%

[Luca Ferrari]

Hi all,
this happened twice to my laptop: kubuntu 16.10, amd64, plain install.
It worked for a couple of months than the kde login screen (progress
bar) freezes at 56% (I got the above from a console) and I'm unable to
do the login.
I reinstalled the distro, and after a couple of weeks, it happened again.
The kde init process seems blocked somewhere (again, I inspect with
top and alike from a terminal). Booting in single mode does not result
in any help.
Any idea on how to investigate/fix?

Thanks,
Luca

P.S.
I know there are too few details, but I need some hint on how to get more