Lighting broken, billboard spheres flattened - what happened?

after saving my project and copying the whole folder to another directory, I somehow managed to get this problem:

For Comparison, this is what it should look like

The backsided and diffuse lighting should not be a problem of individual meshes having inverted normals (even though it looks like we’re seeing the scattered light on the backside of the textures), since those meshes were working just fine before and essentially nothing has changed in those actors.

The “billboard” spheres around actors seem flattened, just as other helper objects (PlayerStart symbol, etc).

What went wrong and how can I fix it?