So far deleting the data cache hasn’t worked for me. This one level I have has done this twice to me now. Very Annoying. What I did to fix it reasonably quickly was use Shift + click to drag and select half the assets in my level using the Outliner list and copy them and then delete them (without saving of course). Then rebuild the lightmass. If it builds you know its an object in the deleted half of the level. Press Ctrl V to paste the half back. Then copy and delete half of the half you first deleted to continue to narrow it down by process of elimination. Hopefully I an easier process can be found soon.