Cannot switch users in Unity

Can't you just log off instead?

I have had problems with black screens when switching users as well on several systems - I have submitted bugs against this problem.

I am fairly confident a fix is in kernel 3.19.0-26. I have a stress test I use (rapidly switching between users) that typically recreates the problem in under a minute. With the new kernel I have on one of my systems done over an hour of this stress test without experiencing problems. I am currently continuing to test on multiple systems and if things keep going well will close out my defects at the end of the week.

Ideally I would like to pull down this kernel as part of the 14.04.3 lts hardware enablement stack. However the commands for this result in dependency errors (and others have reported these failures as bugs).

Fortunately in terms of fixing the problem it doesn't seem the entire stack is needed. So try running the command

sudo apt-get install --install-recommends linux-generic-lts-vivid

This will install the required kernel and once you reboot I think you will find user switching works for you without experiencing black screens.

FYI - my systems are Dells and kernel 3.19 does have new backlight support for DELL. So consider that if you are on different hardware: moving to a new kernel may not solve your particular problem. I'd say it is worth a try. Maybe run 14.04.3 on a live-usb with you system to determine if you want to go forward with updating the kernel.