I am used to this kind of stripe effect in UE4 when using dynamic lighting. It has to do with shadow maps, and you have to trade off shadow accuracy against these stripe artifacts.
This typically happens because the light is too close to a surface (happens constantly for people who try to place lightsources right up against a wall, or its influence radius is too large.
In 5.0 virtual shadowmaps mostly address this issue, but if you make your influence radius high enough, you’ll still get the same problem: