So I have a mod 'Dwemer Egg'. Usually works OK, but one user started getting CTDs.... but only if 6 mods are loaded together:

The Dwemer Egg-Black Marsh-The Reserve-Elvenwood-Moon Path-Royal Mansion

Using both the CreationKit and TESVsnip, all mods look clean. There are a few conflicts, but none between all 6:
NAVI - all of them (but every mod has that)
PlayerBookShelfContrainer - blackmarsh and reserve
A collision marker - blackmarsh and reserve
NobleChest01 - elvenwood/reserve
NobleWardrobe01 - elvenwood/reserve
AlftandWorld - royal/dwemer (neither mod actuall modifies this area, but the CreationKit keeps marked it as edited)

Disabling any one mod fixes the crash. Enabling all six brings the crash back.

Any ideas on how to troubleshoot this further? I have a few other users also reporting CTD, but this is the only one that was reproducible.