Someone reported to me that using Armory-Expansion(1.4.2), Extra Utilities 2(1.9.9), and ToMeTinkers(1.2.3) causes this crash, which i could reliably reproduce.
This seems to be caused by two mods trying to add traits to the same material for the same armor parts.
Because ToMeTinkers adds a modified version of the XU2 traits that actually works on armor, while Armory Expansion doesn't i would suggest disabling Armory Expansions support if both mods are loaded.
I think there would be two ways to fix this:

• ToMeTinkers could disable the Armory Expansion support(I haven't found a way to do this yet, but it should be possible), or
• Armory expansion could check whether a material is already registered for armor parts before trying to register the material(i think this would be the better solution)

That's a major error on my part. I forgot to check if the trait is already registered to the material before trying to do so.
I do already check if an armor part is registered before trying to register said part, but since traits are added on a different section, there's no way to check if the armor part was added by me or not.