It actually doesn't get as far as a crash log here. Forge 2309 still works so I'm reverting to that.

Minecraft log: https://pastebin.com/WXf5xqru
FML-client-latest.log: https://pastebin.com/ftBi1Dgc

Ah, that's... surprising.

A forge pull request that I made broke it. It's easy to fix, but mixin is being a bit stupid here.