You should probably ask in the After Effects forum. It's where those experts hang out.
You will often see it when you have the current-time indicator positioned at the end of the composition or work area, and you have "From Current Time" enabled in the Preview panel, and then attempt a RAM preview. This is because you're already at the end of the previewable duration.
If it is not that, it tends to be a memory issue where the system is saying there is not enough memory to hold 2 frames worth of preview. You might try Edit >> Purge >> All Memory and see if that clears it.