Quadro K1100M: yet another black screen on boot with Ubuntu and prime!

I’m using 346.59 drivers for Kubuntu 15.04 with a Quadro K1100M card.
When I switch through prime-select from intel to nvidia, then after booting I randomly get a black screen about 50% of the times. I can’t but switch to a virtual console by pressing [Ctrl+Alt+Fn] and then reboot. If I’m lucky, then the normal Kubuntu login screen appears, otherwise I get yet another black screen and have to reboot again.

output of nvidia-bug-report.sh follows
nvidia-bug-report.log.gz (198 KB)

EDIT: now, after several updates and new kernel installation, there is no chance to load the desktop environment (sddm) and login: it fails 100% of the times!

Any idea on how that could be fixed? I’m forced to use the intel card, no chance with nvidia

Bump. This is a critical issue: the card is unusable on my laptop!
I can provide any further detail, besides the bug report attached above.
Any help would be highly appreciated!