Minecraft 1.20.1 doesn't start with my modpack and removing fastfurnace fixes the problem
I am attaching the launcher log as it is all I have
launcher_log.txt

I'm having the same issue https://mclo.gs/WCF82By seems like a mixins error

Caused by: org.spongepowered.asm.mixin.transformer.throwables.InvalidMixinException: PRIVATE @overwrite method m_222692_ in fastfurnace.mixins.json:MixinAbstractFurnaceBlockEntity cannot reduce visibiliy of PUBLIC target method

Looks like they're trying to make a public method private. This doesn't work, obviously. So removing it will fix it for now.