I had to change the Derivs1 to Derivs since you called it so many times.
You need an If() statement or an On Error routine to handle division by zero errors. Either skip that step or set the value in the If().
Use F8 to step through the code to see the computation of yscal as xld noted. The error first occurs in the sub rkqs when you run the main sub RungeKutta.
Add the Compile button to the VBE toolbar and compile your code. Your will see that in the Sub odeint, y() is not defined (dimmed). There are other similar problems such as dydx(). Option Explicit is your friend and shows these problems at Compile time. These Compile errors need to be resolved.
e.g.
Sub odeint(ystart() As Double, nvar As Integer, x1 As Double, x2 As Double, eps As Double, h1 As Double, hmin As Double, NOk As Integer, NBad As Integer)
Dim y() As Double
ReDim y(0 To nvar - 1) As Double