system freezes when closing dropbox folder window

[simone]

I have nautilus-dropbox installed, when I close the dropbox folder window (left click on the cross in the upper left corner) the computer freezes (mouse icon moves correctly, but the rest of the screen is still) and the window remains open. This situation goes back to normal pressing the power button for a short time: a window is prompted asking whether I want to shut-down or restart and dropbox window closes, at this point it is possible to resume. Pressing any other button doesn't change anything, except I can access other tty (CTRL-ALT-Fx).
I open dropbox folder by clicking "open dropbox folder" in the dropbox icon (upper right corner of the screen).

I've experienced this bug ever since upgrading to Ubuntu 15.04, never experienced it before. After that time, I noticed this second bug too (affecting dropbox nautilus):
https://bugs.launchpad.net/ubuntu/+source/nautilus-dropbox/+bug/1453655

The bug is fully reproducible and I can give further information if required.