Pathtracer produces lines on custom mesh

Hi!

I’m currently working on a large environment for a short film.
After creating everything in lit mode I wanted to switch to path tracing, for my desired render result.

The path tracer is creating completely random lines on my custom mesh though (not an landscape).
See the editor screenshots here:

pathtraced:

lit:

I tried to import the mesh with frozen transforms, different UVs, more and less topology.
I also tried to remove all lighs - still happens with a single directional light.
There is only a basic grey material assigned, with no normal/displacement.
No Nanite either.
It’s only on that custom mesh - not reproducible with any native Unreal Geometry.

I dont really know what could cause something like this, any kind of help would be highly appreciated!

Thank you guys!