I have a brand new 27" iMac 5K running El Capitan 10.11.5.
The specs are as follows:
iMac (Retina 5k, 27-inch, Late 2015)
Processor 3.3 GHz Intel Core i5
Memory 32 GB 1867 MHz DDR3
Startup Disk Macintosh HD
Graphics AMD Radeon R9 M395 2048 MB
I recently installed After Effects CC 2015 and when trying to launch the app, the screen goes completely crazy with gigantic flickering distorted images of all the icons on the dock, the menu bar and other weirdness. I am unable to even shut down the app or force quit because I can't see anything meaningful on the screen. The only solution is to hard reboot with the button.
I've seen other issues related to El Capitan and the iMac 5K but they are regarding issues with rendering or something inside the app. I can't even get it to open.
Other CC apps such as Premiere Pro work fine.
I see that it's been moved.
What version of AE have you installed? Is it CC 2015 or CC 2015.3?
First thing I would suggest is uninstalling and reinstalling.