Think this has to do with a change in forge in the newest recommended build.
https://hastebin.skyra.pw/rawaqemaqa.nestedtext

Disabling Biome Makeover fixes this issue, so if it isn't Forge, its some kind of conflict.

Thanks - I'll look into this soon.

I can get it to crash reliably with this: https://legacy.curseforge.com/minecraft/mc-mods/just-enough-resources-jer/files/4631691

I'm not sure what the issue is. It looks like a conflict with DefaultBiomesMixin_RemoveSwampTrees.java, causing the mixin to fail when calling: https://github.com/way2muchnoise/JustEnoughResources/blob/8cf16652efe0d038bbf312ecd1eef0d0c9921992/CommonApi/src/main/java/jeresources/api/util/BiomeHelper.java#L19

I spent a few minutes debugging this, but have no idea why it would fail to apply the mixin and crash when fetching a biome from the vanilla registry.

In this case it is a JER conflict, but doesn't look like a JER bug. I cant access the pastebin for this report to compare.

Interesting - thanks for the information

The version here or here (when approved) should fix it.

Luckily the conflicting mixin was actually pointless and marked for removal lol

Ok, yeah, that's much easier than debugging. I honestly have no clue why it crashed. 10.1.3 works perfectly! Thanks for the update, much appreciated.