After I lock my system and my screen goes off (black), it doesn't wake up anymore. As soon as that happens, I have to use my power button to reboot the system, else I will not be able to login.
I tried disabling USB auto suspend, as suggested over here: Ubuntu 14.04 doesn't wake up after screen is locked and blank which did not work for me, unfortunately.
For now I have set the "Turn screen off when inactive for:" to "Never", via All Settings -> Brightness & Lock. Hopefully that will fix it for now, but if anyone knows a proper solution, please let me know :)
EDIT: Setting "Turn screen off when inactive for:" to "Never", does not work! After locking my system, the screen will turn black anyway and I'll unable to login again.
Alright, so xscreensaver seems to be the solution for now. But you have to make sure you set "Blank After" (through the settings of xscreensaver) to a couple of hours or something, because as soon as it goes "Blank", the same issue occurs. I've set it to 600 minutes for now, because I always shutdown my pc after work, but I have to make sure it keeps working while I'm at work so I don't have to force restart it after a meeting for example.