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Which symptoms? "It crashes" is not a symptom description. That's like saying a headache and getting your leg cut off are the same issue, because both are painful. It makes no sense.

Fair enough, let me amend my statement; I experienced the same symptoms after a similar incident, but I don't know the root cause behind either mine or OP's issue. Might be a different cause. All I'm saying is that from a user's perspective it seems (not saying it is) to be the same problem and in my personal case following the solution posted in the thread helped me solve it. OP wrote he already tried reinstalling everything, I'm merely suggesting he deletes his .minecraft folder next time (after having made backups of his worlds ofc) and see if that solves the issue for him, same as it did for me.

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

ok so I got as far as this: public static final DeferredRegister<IRecipeSerializer<?>> RECIPES = DeferredRegister.create(ForgeRegistries.RECIPE_SERIALIZERS, MajCraft.MOD_ID); public static final RegistryObject<ObsidianForgeRecipeSerializer<ObsidianForgeRecipe>> OBSIDIAN_FORGE_RECIPES = RECIPES.register("obsidian_forge", ObsidianForgeRecipeSerializer); What do I need to put in place of ObsidianForgeRecipeSerializer. I know it needs to be a Supplier, but how to I make one for the ObsidianForgeRecipeSerializer? Is it a lambda expression?

Then please don't help... especially not by providing "just do X" solutions. It will not help anyone.