I've recently upgraded to Ubuntu 15.04. I have two displays (laptop + Samsung). At the beginning, both displays worked without any problem (using the nouveau-driver). I've changed to the NVIDIA driver and then the external monitor turns on for 2 or 3 seconds, showing the correct screen-image & resolution, and then turns black. The monitor is listed as active. No mouse nor launch bar are displayed. It is just pitch black. I reverted to nouveau again, but the problem persisted.
I have a Lenovo T510 with Nvidia GT218M. The external monitor is a Samsung Syncmaster. The monitors are detected without problem.
I don't see big errors in the logs. I tried the NVIDIA settings, xconf, removing monitors.xml, etc. but nothing seems to help.
It feels like whatever I do and save, is overridden after a few seconds by another process turning this monitor off again.