This time it wasn't a thumbnail problem
Today I actually saved the problematic sim templates, and their property sets had custom guids in the hairtone field, instead of the normal hairtone guids.
After searching for those guids, these packages were identified:
eb825c3f-494a-e2ef-6f82-8c87081c60e2 - beosb_tylerhanson_*.package, mesh_beos_tyler-hanson240106.package
50c16a8f-4240-e25f-e091-36bb3e1deca0 - kavar_bandana_*.package, mesh_kavar_twisted.package
27441cb5-4b86-e2eb-b17e-b6bb57a1abdc - shinysims_hair_*.package, shinysims_hair_002m_mesh.package
ec4cff6a-48f4-23c6-c405-c7b14d375d56 - sidesweep_hair_*.package, andie_mesh_mh_ofb1_*.package
The package file names were not the original, as these hairs had been color binned by me.
Anyway, it was all fixed after exporting the hairs in BodyShop and re-binning the new packages again