add() needs exactly two arguments. Is this a bug or...?

Long layer chains, using (index -1) to cascade your expressions, can really bring your processor to its knees. It's better if you can come up with a calculation based on the difference in the layer index between the current layer and the "leader" layer. I'm not sure this is exactly correct, but that would make your rotation expression (for all layers except Layer 1), something like this:

leader = thisComp.layer(1);
delay = (index - leader.index)*0.1;
leader.rotation.valueAtTime(time - delay)

You might try that (and a similar expression for scale) and see if that clears up your issues. BTW, when I switched your project to the legacy expression engine, the errors went away, but the performance was still really sluggish.

I don't get any errors when I try it. I assume layer 5 has the same expression (is that correct?), but it doesn't show an error. How long is your string of layers where you're using (index-1) to refer to the previous layer? At this point I'm stumped. It would help if you could post a project file that demonstrates the problem.