Hello JaroMast,
I have a bit more information I would like to gather from you and I have a few troubleshoots you can try.
1.) What is the lightmap resolution set to for the coffin? You can find this by double clicking on your mesh in your content browser to bring up it’s details panel. On the right hand side there is a section called Static Mesh Settings. In that section there is a tab labeled light map resolution. Try setting the resolution on the coffin to something like 512.
2.) Check to make sure you are calling on the right lightmap channel.
I am linking you to our documentation on lightmap resolution and how to Unwrap lightmaps for UE4. There is a wealth of material covering different issues when it comes to lightmaps.
Please follow these troubleshoots and look through our documentation. If you are still having issue after following these steps then please post here with any further questions.