What is the AE version number (down to the last decimal) for both machines?
Are you talking about the exact same project?
What are the specs of the other machine?
What is the OS version (down to the last decimal) for both machines?
Hello, I have the same problem. Did you find a solution? Thank you
It's a sucky bug.
It is targeted to be fixed in the next update.
There is a workaround.
Read Todd K.'s post here: Re: After Effects 2015 won't preview in real time after El Capitan update.
There is another user posting later in that same thread with a second workaround idea.
Thank you Szalam