I also has this bug with “Array index out of bounds” in my project.
It’s random thing in my case, but mostly related to SkeletalMesh, especially if tried enabling Nanite SkeletalMesh causing insta-crash with Array index out of bounds. But there’s some case with SkeletalMesh without Nanite and still has this issue, in that other case I had to delete the affected SkeletalMesh LOD and regenerate them again after upgrading from 5.5.4.
It also there for me when building HLOD (from commandlet outside of Editor, since from Editor causing shader compiler hang and won’t finished) with Nanite enabled, so I had to disable Nanite from my project config file before building HLOD in 5.6.0.
IIRC this “Array index out of bounds” also present in 5.5.0 back then, hopefully they fixed it in 5.6.1, but seems taking forever to drop..