This happens when I launch the game.

Any ideas ?

Can you send the full crash log, please? It should be in your .Minecraft folder.

Aha! Looks like a compat issue with “fastutil“. Can you send them this log, please? Maybe they’ll know how to fix it.

Can you send the full crash log, please? It should be in your .Minecraft folder.

I got the same error and my log is
crash-2021-08-26_11.01.02-fml.txt

Hey, nvm about the issue 😅
@SundownCam will take over, if he want

Did you fix it?

No, I still have no idea who/what 'fastutil' is

Did you fix it?

I was asking @FunMLG.
FastUtil appears to be a mod you have installed.

Did you fix it?

I gave up 😅
And I didn't have a "FastUtil" mod. I have FastFurnace but I don't think it's that.
Wait, I'll try again one more time and tell you if the problem solved itself
BRB

Well, the problem solved itself 🤷‍♂️
No more "java.lang.ArrayIndexOutOfBoundsException: -1" message
I'm sorry for the inconvenience

Very strange

I just tried again as well and it worked, sorry to bother you.

Very strange

I was on Archlinux when I wanted this mod but I didn't try to relaunch my game. I just deleted the mod right after that because I have too many mods and I didn't want to wait so long (again) but some "bugs" are solved by relaunch the game ;) (Like when I couldn't launch the game because my game version had only 2Go of RAM allocated and I tried with 4Go and everything was good)
Anyway, problem solved. Next time someone will have the same problem, just relaunch the game (maybe with more RAM allocated if it's below 8Go allocated) ^^

I don't know but I can close this issue now ?