This happens with and without the card installed in the WCT:
[18:21:09] [Server thread/FATAL]: Error executing task java.util.concurrent.ExecutionException: java.lang.NoClassDefFoundError: net/minecraft/client/resources/I18n at java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[?:1.8.0_111] at java.util.concurrent.FutureTask.get(FutureTask.java:192) ~[?:1.8.0_111] at net.minecraft.util.Util.func_181617_a(SourceFile:46) [h.class:?] at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:668) [MinecraftServer.class:?] at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:387) [ld.class:?] at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:613) [MinecraftServer.class:?] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:471) [MinecraftServer.class:?] at java.lang.Thread.run(Thread.java:745) [?:1.8.0_111] Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/resources/I18n at p455w0rd.wct.sync.packets.PacketSetMagnet.serverPacketData(PacketSetMagnet.java:46) ~[PacketSetMagnet.class:?] at p455w0rd.wct.sync.network.WCTServerPacketHandler$1.call(WCTServerPacketHandler.java:34) ~[WCTServerPacketHandler$1.class:?] at p455w0rd.wct.sync.WCTPacket.func_148833_a(WCTPacket.java:61) ~[WCTPacket.class:?] at org.spongepowered.common.network.PacketUtil.lambda$onProcessPacket$0(PacketUtil.java:157) ~[PacketUtil.class:1.10.2-2221-5.1.0-BETA-2136] at org.spongepowered.common.event.tracking.CauseTracker.switchToPhase(CauseTracker.java:156) ~[CauseTracker.class:1.10.2-2221-5.1.0-BETA-2136] at org.spongepowered.common.network.PacketUtil.onProcessPacket(PacketUtil.java:156) ~[PacketUtil.class:1.10.2-2221-5.1.0-BETA-2136] at net.minecraft.network.PacketThreadUtil$1.redirect$onProcessPacket$zjf000(SourceFile:539) ~[fl$1.class:?] at net.minecraft.network.PacketThreadUtil$1.run(SourceFile:13) ~[fl$1.class:?] at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_111] at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_111] at net.minecraft.util.Util.func_181617_a(SourceFile:45) ~[h.class:?] ... 5 more
and with card in hand:

[Server thread/FATAL]: Error executing task java.util.concurrent.ExecutionException: java.lang.NoClassDefFoundError: net/minecraft/client/resources/I18n at java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[?:1.8.0_111] at java.util.concurrent.FutureTask.get(FutureTask.java:192) ~[?:1.8.0_111] at net.minecraft.util.Util.func_181617_a(SourceFile:46) [h.class:?] at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:668) [MinecraftServer.class:?] at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:387) [ld.class:?] at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:613) [MinecraftServer.class:?] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:471) [MinecraftServer.class:?] at java.lang.Thread.run(Thread.java:745) [?:1.8.0_111] Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/resources/I18n at p455w0rd.wct.items.ItemMagnet.switchMagnetMode(ItemMagnet.java:236) ~[ItemMagnet.class:?] at p455w0rd.wct.items.ItemMagnet.func_77659_a(ItemMagnet.java:209) ~[ItemMagnet.class:?] at net.minecraft.item.ItemStack.func_77957_a(ItemStack.java:162) ~[adz.class:?] at net.minecraft.server.management.PlayerInteractionManager.func_187250_a(PlayerInteractionManager.java:365) ~[lv.class:?] at net.minecraft.network.NetHandlerPlayServer.func_147346_a(NetHandlerPlayServer.java:710) ~[me.class:?] at net.minecraft.network.play.client.CPacketPlayerTryUseItem.func_148833_a(SourceFile:32) ~[jk.class:?] at net.minecraft.network.play.client.CPacketPlayerTryUseItem.func_148833_a(SourceFile:9) ~[jk.class:?] at org.spongepowered.common.network.PacketUtil.lambda$onProcessPacket$0(PacketUtil.java:157) ~[PacketUtil.class:1.10.2-2221-5.1.0-BETA-2136] at org.spongepowered.common.event.tracking.CauseTracker.switchToPhase(CauseTracker.java:156) ~[CauseTracker.class:1.10.2-2221-5.1.0-BETA-2136] at org.spongepowered.common.network.PacketUtil.onProcessPacket(PacketUtil.java:156) ~[PacketUtil.class:1.10.2-2221-5.1.0-BETA-2136] at net.minecraft.network.PacketThreadUtil$1.redirect$onProcessPacket$zjf000(SourceFile:539) ~[fl$1.class:?] at net.minecraft.network.PacketThreadUtil$1.run(SourceFile:13) ~[fl$1.class:?] at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_111] at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_111] at net.minecraft.util.Util.func_181617_a(SourceFile:45) ~[h.class:?] ... 5 more
There is also no chat message when switching between the modes.

I'm using SpongeForge, this error was caught using version 2221, build 2136 and it also happens with the latest, version 2160. Any other information needed?

I usually don't support SF, but this has nothing to do with them. I'll get a fix out asap. TY :)

Still happens in WCT ver. 0.0.9, same error message.

latest is 0.0.10..at any rate i'm testing and testing..just can't reproduce..definitely fixed on my end..I'll be pushing another update soon as there is another bug i've been made aware of

Sounds good, I'll push that update out on our modpack tonight or tomorrow morning and get back with you.

hello..getting ready to push now..just wanted to let you know that I found a couple more spots where I was referencing I18n on both sides..it's client-side only, so that's why these crashes were happening and I've just found a couple more cases where this is happening..in v0.0.11 this issue should be resolved :) thanks for the heads up!

Alright, I'll hold off on pushing our update until 0.0.11 is ready, our update got delayed anyhow. Sorry I couldn't get back to you earlier on it, and thanks for the hard work with it too!

it's already out :) follow me on Twitter (@TheRealp455w0rd) or join my discord to stay up to date - https://discord.gg/9J7yhUP