My sister has a 2013 Routan that has cracked the flex plate, twice in 40k miles.

It has been done under warranty, but she is trying to understand if something is wrong.

BTW: it is basically a caravan with a VW badge...

Any thoughts?

The most likely reason for a a driveplate to crack twice on the same engine is a crank flange with a sharp edge at the perimeter. There should be a break edge/chamfer there.

If you had a chance to look at the broken driveplate with this condition you will see a sharp line around where the flange left a stress riser. Until the dealer tech hand files and polishes away that sharp edge it will continue to break the driveplate.

The other likely possibility for this year vehicle is the dowel pin bores of the trans case are damaged. The assembly plant had a hard time bucking the engine and trans and damaged one or both of the dowel pin bores.

On the caravans the problem is from the dowel pins between the engine and the trans not being lined up properly. With the trans out you can see that the pins do not sit properly in the case of the trans on the ones with repeat problems. The fix for this is a new trans or trans case replacement.