Pretty sure this edit to ShadowProjectionPixelShader.usf (at line 232, as of the 4.17 release in UE4-QA) fixes it:
float4 HomogeneousWorldPosition = mul(float4(ScreenPosition.xy * SceneW, SceneW, 1), View.ScreenToWorld);
becomes
float4 HomogeneousWorldPosition = mul(float4(ScreenPosition.xy, SceneW, 1), View.ScreenToWorld);