Black screen on boot with Nvidia Quadro K1100M on Dell Precision

[giacof]

I'm using current nvidia drivers (nvidia-346) on Kubuntu 15.04:

$ lsb_release -rd
Description: Ubuntu 15.04
Release: 15.04

$ apt-cache policy nvidia-346 nvidia-prime
nvidia-346:
  Installato: 346.59-0ubuntu1
  Candidato: 346.59-0ubuntu1
  Tabella versione:
 *** 346.59-0ubuntu1 0
        500 http://it.archive.ubuntu.com/ubuntu/ vivid/restricted amd64 Packages
        100 /var/lib/dpkg/status
     346.46-0ubuntu1 0
        500 http://developer.download.nvidia.com/compute/cuda/repos/ubuntu1410/x86_64/ Packages
nvidia-prime:
  Installato: 0.8.1
  Candidato: 0.8.1
  Tabella versione:
 *** 0.8.1 0
        500 http://it.archive.ubuntu.com/ubuntu/ vivid/main amd64 Packages
        100 /var/lib/dpkg/status

$ lspci -v
[...]

01:00.0 VGA compatible controller: NVIDIA Corporation GK107GLM [Quadro K1100M] (rev ff) (prog-if ff)
        !!! Unknown header type 7f

Step to reproduce:

- select nvidia card:
  $sudo prime-select nvidia

- logoff or reboot
- the system hangs on a black screen with the message 'starting version 219' displayed on top

Then I can only switch to a virtual console by pressing [Ctrl+Alt+Fn] and switch to the intel card:
  $sudo prime-select intel

Expected behaviour:

The sddm desktop should load and the login window should be displayed.

[giacof (giacof)]

Is anybody dealing with this? This is a critical issue: the Nvidia card is UNUSABLE so far!
Any help would be highly appreciated.