[18:20:39] [Server thread/ERROR]: Error executing task on Server
java.lang.IllegalArgumentException: Payload may not be larger than 1048576 bytes
at net.minecraft.class_2658.(class_2658.java:39) ~[client-intermediary.jar:?]
at net.fabricmc.fabric.impl.networking.server.ServerNetworkingImpl.createPlayC2SPacket(ServerNetworkingImpl.java:45) ~[fabric-networking-api-v1-1.3.12+13a40c6677-3b97aa93efa4c799.jar:?]
at net.fabricmc.fabric.api.networking.v1.ServerPlayNetworking.createS2CPacket(ServerPlayNetworking.java:388) ~[fabric-networking-api-v1-1.3.12+13a40c6677-3b97aa93efa4c799.jar:?]
at net.fabricmc.fabric.impl.networking.server.ServerPlayNetworkAddon.createPacket(ServerPlayNetworkAddon.java:97) ~[fabric-networking-api-v1-1.3.12+13a40c6677-3b97aa93efa4c799.jar:?]
at net.fabricmc.fabric.api.networking.v1.PacketSender.sendPacket(PacketSender.java:113) ~[fabric-networking-api-v1-1.3.12+13a40c6677-3b97aa93efa4c799.jar:?]
at net.spell_engine.network.ServerNetwork.lambda$initializeHandlers$0(ServerNetwork.java:24) ~[��ǰ��-�������桿spell_engine-0.15.12+1.20.1.jar:?]
at net.fabricmc.fabric.api.networking.v1.ServerPlayConnectionEvents.lambda$static$2(ServerPlayConnectionEvents.java:48) ~[fabric-networking-api-v1-1.3.12+13a40c6677-3b97aa93efa4c799.jar:?]
at net.fabricmc.fabric.impl.networking.server.ServerPlayNetworkAddon.onClientReady(ServerPlayNetworkAddon.java:66) ~[fabric-networking-api-v1-1.3.12+13a40c6677-3b97aa93efa4c799.jar:?]
at net.minecraft.class_3324.handler$jai001$fabric-networking-api-v1$handlePlayerConnection(class_3324.java:17018) ~[client-intermediary.jar:?]
at net.minecraft.class_3324.method_14570(class_3324.java:215) ~[client-intermediary.jar:?]
at net.minecraft.class_3248.method_33800(class_3248.java:131) ~[client-intermediary.jar:?]
at net.minecraft.class_3248.md508ebd$polymer-networking$lambda$polymerNet$prePlayHandshakeHackfest$0$0(class_3248.java:549) ~[client-intermediary.jar:?]
at eu.pb4.polymer.networking.impl.EarlyConnectionMagic.lambda$handle$0(EarlyConnectionMagic.java:32) ~[polymer-networking-0.5.19+1.20.1-e57a37eded877f48.jar:?]
at eu.pb4.polymer.networking.api.EarlyPlayNetworkHandler.lambda$onKeepAlive$0(EarlyPlayNetworkHandler.java:108) ~[polymer-networking-0.5.19+1.20.1-e57a37eded877f48.jar:?]
at net.minecraft.class_3738.run(class_3738.java:18) ~[client-intermediary.jar:?]
at net.minecraft.class_1255.method_18859(class_1255.java:156) ~[client-intermediary.jar:?]
at net.minecraft.class_4093.method_18859(class_4093.java:23) ~[client-intermediary.jar:?]
at net.minecraft.server.MinecraftServer.method_24306(MinecraftServer.java:782) ~[client-intermediary.jar:?]
at net.minecraft.server.MinecraftServer.method_18859(MinecraftServer.java:164) ~[client-intermediary.jar:?]
at net.minecraft.class_1255.method_16075(class_1255.java:130) ~[client-intermediary.jar:?]
at net.minecraft.server.MinecraftServer.method_20415(MinecraftServer.java:764) ~[client-intermediary.jar:?]
at net.minecraft.server.MinecraftServer.method_16075(MinecraftServer.java:758) ~[client-intermediary.jar:?]
at net.minecraft.class_1255.method_5383(class_1255.java:115) ~[client-intermediary.jar:?]
at net.minecraft.server.MinecraftServer.method_16208(MinecraftServer.java:742) ~[client-intermediary.jar:?]
at net.minecraft.server.MinecraftServer.handler$enj000$carpet$modifiedRunLoop(MinecraftServer.java:41451) ~[client-intermediary.jar:?]
at net.minecraft.server.MinecraftServer.method_29741(MinecraftServer.java:650) ~[client-intermediary.jar:?]
at net.minecraft.server.MinecraftServer.method_29739(MinecraftServer.java:265) ~[client-intermediary.jar:?]
at java.lang.Thread.run(Thread.java:833) ~[?:?]

Please attach a full logfile.

0.zip

I am sorry to say your modpack is too large.
The 1.20.1 game version of Spell Engine, does not support such huge modpack.

Not big, I only added 800 mods with items, I haven't added mods without items yet

Regardless of personal opinions on whats too large, Spell Engine cannot handle that much data, due to the limitations of Vanilla packet system.
Currently, there is no development capacity to return to a legacy version for rewriting the entire synchronisation stack.