Why there are black lines on my wall?
And how can I solve this?
There can be many things wrong here few that come in mind: lighting UVs, material, a problem with the mesh itself, clipping of another mesh, too far from the origin point of level(0,0,0) physics and graphics get janky when you get to further than 200,000 from the center with I doubt you had. So I really think UVs start with that move the mesh a little bit show its UVs and faces surely there we will find whats wrong.
Did you ever solve this one mate? I am using the same dungeon pack and getting the black lines. Real shame because I think its an issue with the pack but I do like the assets.