2 days ago everything was fine but since today i cant start the game anymore with the AbyssalCraft mod active.

The game crashed: initializing game
Error: java.lang.RuntimeException: Biome ID 100 was overridden by the biome WitchwoodForest (am2.worldgen.BiomeWitchwoodForest)!

This is the crash report i get
This Is on Minecraft Version 1.7.10

It’s a biome ID conflict, find whatever the “witchwood forest” is in configs and change its id to something other than 100. Alternatively find whatever abyssalcraft biome in abyssalcraft’s config is 100 and change that to something other than 100. Also why tf are you playing abyssalcraft in 1.7.10 it absolutely sucks in that version.

Problem is i cant see no witchwoodforest in the files nor any biomes o'plenty biome id the same as Abyssal.
Since it seems to have only problems with that one mod

And the reason playing this version is bc nostalgia bc of a Minecraft series from 2015 with a modpack that has AbyssalCraft in it

Looks like that's a biome from Ars Magica 2 (am2.worldgen.BiomeWitchwoodForest).
AbyssalCraft uses the ID range 100 - 113, so if any other Ars Magica 2 has any other biomes, make sure their IDs are outside that range.

Looks like that's a biome from Ars Magica 2 (am2.worldgen.BiomeWitchwoodForest).
AbyssalCraft uses the ID range 100 - 113, so if any other Ars Magica 2 has any other biomes, make sure their IDs are outside that range.

Can i use any number id or is the a specific range? (im not very experienced with this)

I think Biome IDs in 1.7.10 is limited to 255, but I have no idea which biome-adding mods you have installed, nor which IDs they occupy.
This is pretty much trial and error until the game starts. I would start by checking the Ars Magica 2 config file (would likely be called am2.cfg), then change all biome ID options in there to 114 and increment the number if there's more than just this one biome.

I think Biome IDs in 1.7.10 is limited to 255, but I have no idea which biome-adding mods you have installed, nor which IDs they occupy.
This is pretty much trial and error until the game starts. I would start by checking the Ars Magica 2 config file (would likely be called am2.cfg), then change all biome ID options in there to 114 and increment the number if there's more than just this one biome.

Ars Magica had only that biome in the config.
Changed it to 114 like you said now it works again
Thanks for the Help