At first, I derived that:
$$\nabla \frac 1{\mu}=-\frac 1{{\mu}^3}\left((1-\beta^2)+\frac{\dot{\vec\beta}\cdot\vec R}c\right)\vec R$$
(dot means differentiation with respect to ##t'##).
I assume this result is true because it gives valid result for magnetic field.
To find electric field one should also derive partial derivative of ##\vec A## with respect to ##t##. I've used chain rule, substituted ##\vec A## and used derivative of product formula.
$$\frac {\partial \vec A}{\partial t}=\frac...