Multiple lights shading problem

The problem is well visible on that screenshot (in-game view):

Each of the torches has an attached light source, as well as an orb in the middle. This creates visual artifacting on the floor. The floor is made out of multiple squares, and all the artifacts are located specifically on the borders of those tiles. More specifically, the tiles are not shaded in a seamless manner, but the borders are very well visible.

The problem is not caused by shadows (checked with all the shadows disabled), all materials are default.

Same thing happens in the inspector, when the lights are turned up to a higher range value:

What can be a cause of that problem?