Hello SiCzech,
There is a known a reported issue with the lightmaps of the walls and floors for the blueprint template projects being set too large and causing Swarm Agent and light building to fail or crash. If you are able to reproduce your issue in a blank template project as mentioned, then follow the steps below to fix this issue.
Disable Lightmap Resolution Override
Below you will find the actual Lightmap Resolution for the ‘CubeMesh’ within the ‘Static Mesh Editor’
Static Mesh Lightmap Resolution
Here you can set the value to something like 128 or 256 to get better performance and faster light build times. Let me know if these suggestions helped resolve your issue, or if you need further assistance .
Cheers,