I can launch VirtualBox but any attempt to open VM’s settings or run VM terminates my session and I’m presented with the login screen. This happens with old (and previously working) VMs and brand new ones.
Any clue will be appreciated 
Version details: VirtualBox 4.2.16-85.5 from
Index of /repositories/Virtualization/openSUSE_12.3
OpenSUSE 12.3 x86-64 desktop kernel
KDE 4.10.5
How much memory do you have and how much to the VM?
I recently had this issue as well, which updating the the Nvidia proprietary binary driver corrected. Might that apply in your case?
Indeed, reinstalling the NVIDIA driver fixed it. Thank You.
Box has 16GB RAM and VMs use between 512 MB and 2 GB. I don’t run more than 1 VM at a time.
That is plenty. I guess the NVIDIA driver reinstall worked so all is good