Alright I creating a modpack, with around 300 mods. (A little under it but it doesn't matter)
And I came across this when i wanted to check that the Chests Loot is Okay. But then it doesn't open it. I looked at the logs and found this
I can't see what causes it.
I am using the newest Forge Version. (184.108.40.20672)
Edit: Also tested it on the recommended (2768)