Sounds like it might be getting too much sun. A variegated shrub like this needs protection from too much sun, especially hot afternoon sun. If you have it on the west side of your house, it may be getting more than it can stand (depending on whether you have big trees over there providing shade).

I can tell you right now, that afternoon shade may not be enough. I don't know what the right spot is for this shrub, but mine is planted in a spot that's fairly sheltered to the west and south and it has died off every year since I planted it. I have another variegated elderberry (green and white, don't know the cultivar name) that's in house shade all afternoon and it is also turning crispy, though it is doing somewhat better than 'Madonna'.

Here's a monkey wrench in this one - i have a Madonna in full sun, its not turning crisp as you've described - its just not that vigorous. Its been in the ground 3 or 4 years, died all the way back one winter, is only a foot and a half tall. It may do better in a filtered sun situation - i have a Sutherland gold elder that holds color much better in filtered sun.

(offtopic - a local nursery here has variegated cornus mas (cornelian cherry dogwood) that are really striking!)