It's a hiccup in AE's internal memory caching system. Sometimes viewing a layer/ pre-comp before it has been time-remapped will fill certain buffers and after the time-remap they are not invalidated, because certain frames/ fields still coincide with the same point in time. It's pretty arbitrary, but usually restarting the program cleans out any garbage and makes it work...
Before restarting you can also try to purge memory (Edit > Purge > All) most of time it works for such problems.
It's worth mentioning... That also you have to be careful where are you trying to ram preview the clip. If you are trying to ram preview the clip at the layer itself in the comp window instead of the whole comp, you'll get this message. So always check up you are in the master comp, where you applied the remap, and not inside of it.
Check your tabs!