I know there are already reports such as this one, but none of those helped me. When I click on the button, in my multiplayer server console it says:
[22:32:37] [Server thread/FATAL] [minecraft/MinecraftServer]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NoSuchMethodError: net.minecraft.network.play.client.CPacketCloseWindow.<init>(I)V
at java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[?:1.8.0_212]
at java.util.concurrent.FutureTask.get(FutureTask.java:192) ~[?:1.8.0_212]
at net.minecraft.util.Util.func_181617_a(SourceFile:47) [h.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:842) [MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:464) [nz.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:786) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:667) [MinecraftServer.class:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_212]
Caused by: java.lang.NoSuchMethodError: net.minecraft.network.play.client.CPacketCloseWindow.<init>(I)V
at org.bukkit.craftbukkit.v1_12_R1.event.CraftEventFactory.callInventoryOpenEvent(CraftEventFactory.java:790) ~[CraftEventFactory.class:git-HEAD-a8543ec]
at net.minecraftforge.fml.common.network.internal.FMLNetworkHandler.openGui(FMLNetworkHandler.java:107) ~[FMLNetworkHandler.class:git-HEAD-a8543ec]
at net.minecraft.entity.player.EntityPlayer.openGui(EntityPlayer.java:2732) ~[aed.class:?]
at net.mcreator.eas.MCreatorEasthunderstormCommandExecuted.executeProcedure(MCreatorEasthunderstormCommandExecuted.java:42) ~[MCreatorEasthunderstormCommandExecuted.class:?]
at net.mcreator.eas.MCreatorEASGUI$GUIButtonPressedMessageHandler.lambda$onMessage$0(MCreatorEASGUI.java:197) ~[MCreatorEASGUI$GUIButtonPressedMessageHandler.class:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_212]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_212]
at net.minecraft.util.Util.func_181617_a(SourceFile:46) ~[h.class:?]
... 5 more
Forgot to add. In single player it works just fine.