I had changed the memory and cache settings, because AE told me to: it told me to decrease max RAM cache size, enable disk cache and increase max memory usuage. This stops it from coming up with that error message.
However, I now have a new error message when I try to RAM preview: After Effects error: RAM Preview needs 2 or more frames to playback.
I have a 6 second comp with a 6 second video clip in it.
So, I'm stuck! Note that this is the first time I have used high def stuff at 1980 by 1080.