After upgrading to one of the beta El Capitan builds, I can no longer use the "start screensaver" hotcorner. Other hotcorners still work, so it's not an issue with an application grabbing mouse input and blocking hotcorners from being detected (for example, I can configure the display to turn off and that works fine) and the screensaver itself works, too.
This is on El Capitan public build 15B17c, and has persisted through the few beta builds. Moving the mouse to the corner assigned to "start screensaver" simply has no effect. I've tried disabling and enabling the option, rebooting, checking disk permissions, etc. to no avail. Any other suggestions?
Best Answer
What worked for me:
1) Change the hot corner action to something different. It should work.
2) Restart your Mac
3) Change the hot corner back to the original action.
I just checked that the action keeps working after a restart.
I'm on MBP 15'' Retina Mid-2014 with El Capitan.