Hi there.
I'm currently investigating a startup crash issue that was also posted on your Curseforge page for the mod. https://legacy.curseforge.com/minecraft/mc-mods/onetwenty-1-20-backport/issues/5

Both mods for some reason got the same problem in how items are registered, the layers for the cherry sign does overlap and for some reason the mod idea gets read as "minecraft:" which is not what you want, right? (I saw that both mods, according to Deobf in Gradle, have a file that defines the modid somewhere but maybe you missed out on one specifically important place. Either way, mapping it on "minecraft:" is error prone and not good. Even if accidentally, don't do that.)
I dug a bit through the code (I dunno how long this takes on your end, Regions Unexplored dropped all support.)
I am not an expert, but using mixins to change parameters sounds like an uncool thing to do, performance wise.

The code snippet I found through dependencies was in your Moditems.class https://github.com/FrostBreker/OneTwentyBackport/blob/20b2e75c29a5b093b996203a0e78f7393e65da1e/src/main/java/fr/frostbreker/onetwenty/init/ModItems.java#L13C3-L13C120
where you registered with the deferred register

public static final DeferredRegister<Item> ITEMS;

MAYBE change it to this instead.
public static final DeferredRegister<Item> ITEMS = DeferredRegister.create(ForgeRegistries.ITEMS, "onetwenty"); <- the mod id needs to be your mod's. I think.

I am NOT sure if this does anything. But if it helps fix the issue and you actually get to it faster than I with my whacky coding skills (because if not I need to write a Mixin class with this and I don't even know much about Mixins or MC modding at all), I would be grateful. https://docs.minecraftforge.net/en/1.18.x/concepts/registries/ here some more info on registries.

I already tried to cancel the second registrykey from being generated and whichever of the double items to get swallowed, (possibly adding it with some tricks in kubejs later to mend the hole), but ha the code I tried didn't cancel anything at all. Idk if Forge did something whacky and blocked the registry intervention or it was called too early/late.

I'm ok with life's happening and all, just please let me know if a) you are ever going to fix this, or trying to b) if you maybe got an other idea why Regions Unexplored is using the same layer for your "minecraft:cherry_sign" as the crash log states each time. Just anything, a sign of life so I can use both mods together in my modpack. Or go cry and abandon it if my Mixins as a last resort won't work. I thought I share the info I got together at least, in hope that raises the probability of the problem getting solved at its root. Thanks in advance.