AgriCraft/AgriCraft#1205
Basically, it messes with an import.

Is the mod Open Source?

Yep!
It's getting rewritten, but the branch we're currently using is FD/OldPackets.
https://github.com/ProjectKorra/AvatarMod/tree/FD/OldPackets

This is what's crashing:
https://github.com/ProjectKorra/AvatarMod/blob/FD/OldPackets/src/main/java/com/crowsofwar/avatar/client/render/RenderArc.java

Strange, can it be that it is because InfinityLib ships joml as well?

Perhaps try updating to the same version of joml?
https://github.com/ProjectKorra/AvatarMod/blob/FD/OldPackets/build.gradle#L44-L46

The stacktrace displays this:
java.lang.NoSuchMethodError: org.joml.Vector4d.mul(Lorg/joml/Matrix4d;)Lorg/joml/Vector4d;

I have no idea what that 'L' is doing there...

Ok I'll try doing that! I have 4 days off school coming up :P
Yeah I don't know why either 😂

Hey, the other developer of the Avatar mod here.

I have just resolved this issue on our side, by relocating the joml and snakeyaml packages to the com.crowsofwar.avatar.<module> package. I suggest you do this too to prevent further issues like this. Feel free to take a look at our build.gradle to see how I did it. I relocated them using the shadow plugin, which I really recommend. It is important to not forget to reobfuscate the shadowed jar. Again, see our build.gradle for an example on how to do that.

Cheers!

Closing since I am moving to 1.16 where this is no longer relevant (I am no longer using joml).