using WorldSalad(tried every v1.0.2-1.0.4)
and geckolib(tried every v3.0.46-3.0.0)
with JRE_8 (tried both v51-333)

Forge version 31.2.57, for MC 1.15.2 with MCP 20200515.085601
server log:
[Server thread/INFO] [net.minecraft.server.dedicated.DedicatedServer/]: player joined the game
[Netty Server IO #2/ERROR] [net.minecraft.network.NettyPacketEncoder/]: java.lang.ArrayIndexOutOfBoundsException: 1
[Netty Server IO #2/ERROR] [net.minecraft.command.arguments.ArgumentTypes/]: Could not serialize net.minecraftforge.server.command.ModIdArgument@46f7c32f (class net.minecraftforge.server.command.ModIdArgument) - will not be sent to client!
[Netty Server IO #2/ERROR] [net.minecraft.command.arguments.ArgumentTypes/]: Could not serialize net.minecraftforge.server.command.EnumArgument@6d4c777d (class net.minecraftforge.server.command.EnumArgument) - will not be sent to client!
[Server thread/INFO] [net.minecraft.network.play.ServerPlayNetHandler/]: player lost connection: Internal Exception: io.netty.handler.codec.EncoderException: java.lang.ArrayIndexOutOfBoundsException: 1
[Server thread/INFO] [net.minecraft.server.dedicated.DedicatedServer/]: player left the game

sometimes different
[Server thread/INFO] [net.minecraft.network.play.ServerPlayNetHandler/]: player lost connection: Internal Exception: io.netty.handler.codec.EncoderException: java.lang.ArrayIndexOutOfBoundsException

client/server mods list (/forge mods):
• minecraft forge-1.15.2-31.2.57-client.jar : minecraft (1.15.2) - 1
• mods_folder WorldSalad-1.0.4.jar : worldsalad (1.0) - 1
• maven_libs forge-1.15.2-31.2.57-universal.jar : forge (31.2.57) - 1
• mods_folder geckolib-forge-1.15.2-3.0.46.jar : geckolib3 (3.0.46) - 1

bonus: discord link expired, unknown fandom/github wiki...