There's a whole thread on this already, please look it up. It seems to be a more general issue, especially on Mac.
Here are two links for other dthreads about this - as I said in the one, a simple reboot solved my issue...
I have also read somewhere that some people finding that restarting just After Effects sometimes helps as well.
The After Effects CS6 (11.0.2) update was just released. It includes many bug fixes and adds some GPUs to the list of those that can be used for GPU acceleration of ray-traced 3D rendering.
Details are here: http://adobe.ly/AE1102
Note that one of the changes is the addition of a new Memory & Multiprocessing preference and an adjustment to memory handling on Mac OS to alleviate the problem seen when getting the message “After Effects error: RAM Preview needs 2 or more frames to playback”.