"Start Screensaver" hotcorner no longer works under El Capitan beta
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.