Just wanted to post my experience with this and hopefully it helps someone else.
We spent a number of days trying to track this down. We’d made a number of houses out of modular parts and then wanted to test HLOD with it before going too far. What we found was that some of the parts we had used were using LOD0 for HLOD0 and some were using LOD1 (or even LOD2) for HLOD0. This led to the raw mesh is corrupt error because the number of UVs in each channel didn’t always match the number of WedgeIndices. Once we adjusted the LOD screen size of LOD1 to ensure it was above our HLOD0 screen transition size the message went away.
TLDR; Ensure that the models that are in a cluster are using the same LOD level for generation. If the LOD level differs you’ll receive a corrupt mesh error message.