Ubuntu – Screen turning black after switching users

I've create three users on my computer (admin + standard users), and whenever I switch between users, my screen is turned off.

I have Ubuntu 16.10, and I switch users using the account menu on the top right corner.

Symptoms

I have two standard users who can login without a password. When I switch between them:

• Instantly and for a brief second I see the screen of the other user.
• The screen turns off (the LED indicator on the screen starts blinking).
• Now there are two options:
  • The screen turns back on after a 1-3 seconds.
  • The screen stays off until I move the mouse or click on a key, as if the computer is suspended.

Additional Symptoms

When I switch to the administrative user, it is more complicated:

• The screen blinks for a second.
• I get the display manger where I need to enter my password.
• The administrative user might not be selected – I have to choose it again, and enter the password.
• I am logged in and see the screen for a brief second.
• The screen blinks.
• I see the display manager a second time. This time there isn't a list of users, only the administrative user. I enter the password again and log in without blinking.
• I see an error message:
  

Things that didn't work:

• I tried clicking on Ctrl+Alt+F7 to switch between TTY sessions. This sometimes works, but most of the times the screen still turns off.
• I tried dm-tool switch-to-user username. This is the same as clicking the menu.
• I tried changing the power-saving options to disable suspend mode (screen never turns off when inactive). This had no effect.
• I tried switching the display manger to gdm3. This was terrible. The system wouldn't start and I had to use root console from grub to restore lightdm.
• dpkg-reconfigure lightdm.
• I tried using an HDMI cable with my graphics card (Gigabyte RX 460 2G 1212Mhz WindForce2 OC), or a VGA cable to my motherboard (ASUS H110M-K DDR4). Both behave the same.
• I didn't try installing amdgpu-pro – it seems to support only 16.04, and isn't needed for 16.10 (which has built-in support).

Similar questions with no answers:

• Black sreen after switching user
• Blank screen after Switch User or Resume
• Ubuntu 16.04 LTS – Black screen after switching users

I will be switching users very often… How can I get this to work smoothly? Is there a log I should check?

I've found this on apport.log:

ERROR: apport (pid 19659) Fri Dec 2 12:22:40 2016: called for pid 19445, signal 6, core limit 18446744073709551615
ERROR: apport (pid 19659) Fri Dec 2 12:22:40 2016: ignoring implausibly big core limit, treating as unlimited
ERROR: apport (pid 19659) Fri Dec 2 12:22:40 2016: executable: /usr/lib/xorg/Xorg (command line "/usr/lib/xorg/Xorg -core :3 -seat seat0 -auth /var/run/lightdm/root/:3 -nolisten tcp vt10 -novtswitch")
ERROR: apport (pid 19659) Fri Dec 2 12:22:40 2016: is_closing_session(): no DBUS_SESSION_BUS_ADDRESS in environment
ERROR: apport (pid 19659) Fri Dec 2 12:22:40 2016: this executable already crashed 2 times, ignoring

I think this is related because sometimes after switching users I see the error message, and sometimes hear the error message sound.