Typically this error would come up with expressions when attempting to e.g. access a wrong index for an item, but since the tutorial doesn't seem to use any, I'm at a loss. The only thing that is apparent in your screenshot is the negative offset on one of your layers, which may somehow wreak havoc with one of the effects used. Perhaps you overcranked the values on the preceding key and suddenly going negative throws things for a loop. In such a case it would be best to simply delete the keyframes or even the entire layer and start from scratch.