I was just playing around and when I joined my server on 1.12.2 (my client was on 1.15.2, I turned that option on, so I can join it), everything worked fine. The resource pack installed correctly. But when I got kicked, the game crashed. I have 2 other mods installed, the name and links are:
OptiFabric (https://www.curseforge.com/minecraft/mc-mods/optifabric)
OptiFine (http://optifine.net/adloadx?f=preview_OptiFine_1.15.2_HD_U_G1_pre18.jar)
Here is the crash report:
crash-2020-05-31_08.36.37-client.txt

Also, it might not matter, but my server is running git-Spigot-2f5d615-aae46f8 (MC: 1.15.2) (Implementing API version 1.15.2-R0.1-SNAPSHOT) and I have ViaVersion with ViaBackwards and ViaRewind.

If you need any more information, you can just ask me.

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Is there a server resource pack? What resource pack were you using? Mind sending the whole log?

The error doesn't come from multiconnect because it has nothing to do with the protocol, so I think it surely comes from optifine/optifabric.

The problem may well relate to multiconnect, the stack trace seems to suggest that the item registry is being modified while being iterated over by optifine (while resources are reloaded) after he got kicked. Multiconnect changes round the item registry when the protocol changes. In fact, it might be possible to reproduce without optifine, I am just waiting on more info from him.