FIXED! I reinstalled Tasker (also deleting remaining Tasker folder) and now it works as it should. Not a clue why it didn't work at first.
I made a little program in tasker. Two tasks: activate stamina mode, activate normal mode. The first is activated when the screen is turned of, the other when it's turned back on. For this I used the events "Display off" and "Display on".
Now I tried the 'Display state' state and set it to "off". Then added the 'activate stamina mode' task to its 'enter'. Also added 'activate normal mode' to the exit. Though for some reason this doesn't work.
So my question is, why?
Note: Using Cyanogenmod 10.1 on Xperia Z. Tasker version v4.0u1
Edit: I know it doesn't work because I added a notification & buzz to the task, these don't get fired. I also tried to add an exit and enter to the running of an app, and this DOES work. Very weird :s