Since the Bug Submission Form does not support uasset as attachment, here a bug report.
BUG REPORT
After upgrading a project from 4.27 to 5.0.3 a level stopped loading, a pop-up window states Fatal error. The logs are not really helpful.
I was able to track the culprit down to a specific merged mesh (see attachment). Perhaps a material from the mesh may be problematic, but in any instance the level should keep loading if a mesh is problematic.
SM_Base_Building_03.uasset (769.5 KB)