Primitive mobs loads incorrectly JEI and cause the game to crash when NEI is present.
TheCBProject/NotEnoughItems#139

I will try to take a look at it. However I'm not doing anything unconventional as far as I know. I would need to know what JEI is trying to accomplish. It tries to get an ingredient type in my VillagerProfessions registry, which is confusing. Will test.

Rather than NEI this has to do with JEI Villagers. The Travelling Merchant overrides the getTrades method to choose random trades each time. I'll try to see what I can do; but might not be able to fix it soon. Just Enough Resources copes with my custom trades just fine.

EDIT: Ok nvm, this has to do with a custom ITradeList I made so I can trade 2 items for 1 emerald in a Villager trade. Seeing as it extends ITradeList I'm not necessarily in the wrong; however JEI Villagers cannot seem to cope with the class type. I might look at this some time, but I suggest you also post this in the JEI Villagers issue tracker. JER seems to cope with the trades just fine, so you could also use that instead.

Removing the TradeList still results in a crash. So it is probably the Traveling Merchant anyway. I cannot remove the override of getTrades() for my Traveling Merchant. I need it for the random trades. I will close this as this is not going to be fixed unless the JEI Villagers dev does a similar check (maybe just a null check?) as Just Enough Resources, which does not crash using my Villagers (it just shows no trades for the Traveling Merchant and the Lost Miner is fine).

I investigated this and it was actually an issue in JEI, surprisingly. I fixed it in the latest version.
mezz/JustEnoughItems@ce6aac1

Did not expect that. Thanks, mezz!