It's a bug with how the new architecture (introduced in 13.5) works with El Capitan (whether the bug is Apple's fault or Adobe's is irrelevant - it's a highly annoying bug if you don't know about the workaround). There have been a number of threads here about it. And it's been mentioned in the Adobe After Effects blog in a couple of posts.
As I hinted previously, there is a very simple workaround.
Dock another panel under your timeline panel. Like this:
Note how the Render Queue is docked under the timeline panel.
This will enable you to get cached previews that play back in real time with both spacebar and numpad 0 previews. If you would rather keep your render queue somewhere else, pick another panel you care about less.
Alternatively, you can apply to beta test the 13.7.1 bug fix update which is meant to fix that bug: help test the After Effects CC 2015 (13.7.1) patch update
No problem! Glad to help.