Ubuntu 14.04 doesn't wake up after screen is locked and blank

When I lock the screen (Super+L or Ctrl+Alt+L) and wait until the screen goes dark, I can't resume anymore. I move my mouse and press my keyboard, but screen stays blank. My monitor goes into standby mode, if I turn it off and on again it says "no input".

I suppose that pressing any keyboard key or moving the mouse doesn't wake it up because the keyboard is not working. The caps lock and num lock buttons don't light up when I press them (they usually do). Why is the keyboard turned off when screen is locked?

It's Ubuntu 14.04 on a DELL Optiplex GX620, the mouse is wireless (Logitech Unifying Receiver) and the keyboard is connected via USB. I tried connecting a different mouse or keyboard to a different port. The mouse's red laser at the bottom does light up, but the PC does not resume.

I have used Debian Sid, Arch Linux and elementaryOS on this computer before Ubuntu, and this problem did not occur. Ubuntu is the only distro which locks the screen with LightLocker, so maybe the problem is with LightLocker...

The weird thing: This does not happen if I leave the PC alone and wait until it locks itself automatically! Only if I manually lock it.

Edit: I can't access TTY1-6 either.

I had similar issues with Ubuntu 14.04 and 16.04 more recently.

After struggling for a while, I found that disabling USB auto suspend, solved (worked around) the problem.

This feature is a part of usbcore which is compiled in the kernel in Ubuntu.

From https://unix.stackexchange.com/a/175035,

Edit the /etc/default/grub file and change the GRUB_CMDLINE_LINUX_DEFAULT line to add the usbcore.autosuspend=-1 option:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash usbcore.autosuspend=-1"

Note that quit splash were already present options. So keep other options you have too.

After save the file, update grub:

sudo update-grub

And reboot.

Now check autosuspend value:

cat /sys/module/usbcore/parameters/autosuspend

And it should display -1.