plasma on 15.04 unusable

[Luca Ferrari]

Hi all,
after succesfully upgrading the 14.10 to 15.04 I'm not able to run
plasma desktop at all.
Each time I log in the disk starts working at full, I can see from a
console that tracker-store and other tracker-* processes are running.
Also a baloo_file is running. After a few minutes the system is still frozen.
If I kill such processes I can resume the graphical workspace but
without any window decoration.
If then I leave and enter again the desktop I cannot go other than the
splash screen, and the disk is doing nothing at all.
I'm running KDE Framework 5.12.

Any idea?

Thanks,
Luca