Issue (The better the description the better i can solve the issue):

Game crashes on launch citing "java.util.ConcurrentModificationException: null". Looks maybe like a mod incompatibility. Mod list is in the crash log.

Steps to reproduce:

Install the exact same mods as me and launch the game I guess?

Affected Versions (Latest is NOT a version):

• Minecraft: 1.16.5
• Forge: 36.1.15
• TenshiLib: 1.3.0
• ImprovedMobs: 1.7.6

Logs (Use a site like pastebin, gist etc. DONT COPY PASTE THE LOG IN HERE): https://pastebin.com/ZTWvQS2r

crash-2021-05-15_22.31.27-client.txt

does it happen with only this mod? and can you show the config file?

Just gave it another whirl and it didn't happen this time, Improved Mobs seems to be working nicely now. I had made a small change to the mod list since then (https://www.diffchecker.com/bT3miaHu), but I went back to the same set of mods I had then to test and it still works. I realize that this probably isn't very helpful, sorry about that. If it happens again I'll comment here.

The three config files (I don't remember changing anything): https://pastebin.com/dQpUBT8S

this is a really strange CME. did that really occured during loading? and not while in a world?

Yeah iirc the crash happened early in Forge's initial mod loading routine, before getting to the MC main menu.