When the login screen appears, only the left half of the screen is shown on the primary display. The other half seems to be displayed on my beamer also attached to the graphics card but usually not running and also not active in Plasma's screen setup.
The bug seems to be discussed also here:
https://github.com/sddm/sddm/issues/265
ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: sddm 0.11.0-0ubuntu10
ProcVersionSignature: Ubuntu 3.19.0-18.18-generic 3.19.6
Uname: Linux 3.19.0-18-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.17.2-0ubuntu1.1
Architecture: amd64
CurrentDesktop: KDE
Date: Mon Jun 1 21:08:48 2015
InstallationDate: Installed on 2013-11-29 (548 days ago)
InstallationMedia: Kubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
SourcePackage: sddm
UpgradeStatus: Upgraded to vivid on 2015-05-25 (7 days ago)
Status changed to 'Confirmed' because the bug affects multiple users.