Hello there @kabisouth!
Form your screenshot, it seems UE is casting the shadows from the leaf cards themselves, the component from which the tree’s foliage is generated. It also reminds of a quite similar case:
The solution there was to switch the shadow method being used. In your scene, go to Edit > Project Settings > Engine - Rendering, and look “Shadow Map Method”. If it’s currently set on Virtual Shadow Maps, then change it to Shadow Maps, and viceversa.
The other element is the combination of Nanite and Ray Tracing. Having them all actived together can generate this effect in foliage. Experiment by disabling one first, then the other, then both. These changes are also done via Project settings, via the “Ray Traced Shadows” check, as seen above, and the Nanite check, also contained in Rendering:
