Skript version: v2.2-dev23.
Mc version: Spigot 1.8.8 (with ViaVersion)

Console error, caused by WildSkript's change skin effect.

Spoiler: Error

7.02 09:58:02 [Server] ERROR Could not pass event PlayerJoinEvent to Skript v2.2-dev23 17.02 09:58:02 [Server] INFO java.lang.NoClassDefFoundError: net/minecraft/server/v1_7_R3/EntityHuman 17.02 09:58:02 [Server] INFO at net.dzikoysk.wildskript.util.SkinUtils.change(SkinUtils.java:25) ~[?:?] 17.02 09:58:02 [Server] INFO at net.dzikoysk.wildskript.effects.EffSkin.execute(EffSkin.java:22) ~[?:?] 17.02 09:58:02 [Server] INFO at ch.njol.skript.lang.Effect.run(Effect.java:54) ~[?:?] 17.02 09:58:02 [Server] INFO at ch.njol.skript.lang.TriggerItem.walk(TriggerItem.java:65) ~[?:?] 17.02 09:58:02 [Server] INFO at ch.njol.skript.lang.TriggerItem.walk(TriggerItem.java:93) ~[?:?] 17.02 09:58:02 [Server] INFO at ch.njol.skript.lang.Trigger.execute(Trigger.java:53) ~[?:?] 17.02 09:58:02 [Server] INFO at ch.njol.skript.SkriptEventHandler.check(SkriptEventHandler.java:152) ~[?:?] 17.02 09:58:02 [Server] INFO at ch.njol.skript.SkriptEventHandler$1.execute(SkriptEventHandler.java:112) ~[?:?] 17.02 09:58:02 [Server] INFO at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at org.bukkit.plugin.TimedRegisteredListener.callEvent(TimedRegisteredListener.java:36) ~[spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at net.minecraft.server.v1_8_R3.PlayerList.onPlayerJoin(PlayerList.java:298) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at net.minecraft.server.v1_8_R3.PlayerList.a(PlayerList.java:157) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at net.minecraft.server.v1_8_R3.LoginListener.b(LoginListener.java:144) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at net.minecraft.server.v1_8_R3.LoginListener.c(LoginListener.java:54) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at net.minecraft.server.v1_8_R3.NetworkManager.a(NetworkManager.java:231) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at net.minecraft.server.v1_8_R3.ServerConnection.c(ServerConnection.java:148) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:814) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at java.lang.Thread.run(Thread.java:745) [?:1.8.0_112] 17.02 09:58:02 [Server] INFO Caused by: java.lang.ClassNotFoundException: net.minecraft.server.v1_7_R3.EntityHuman 17.02 09:58:02 [Server] INFO at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:91) ~[spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52] 17.02 09:58:02 [Server] INFO at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:86) ~[spigot-1.8.8.jar:git-Spigot-e4d4710-e1ebe52]

I know for sure this is WildSkript's change skin effect. As soon as I added this code, the error began to occur whenever a player joined. Also, looking in the error message, it seems like it has to do with A. WildSkript, B. Player Skins, and C. SkinEffects.
(at net.dzikoysk.wildskript.util.SkinUtils.change(SkinUtils.java:25) ~[?:?] 17.02 09:58:02)
(at net.dzikoysk.wildskript.effects.EffSkin.execute(EffSkin.java:22) ~[?:?] 17.02)

Code (Skript):

1. on join:
2.     change player's skin to "%name of player%"

I was making a skript allowing players to change their skin, which is why I was using WildSkript's effect.

Addons:
SkQuery v3.21.4, SkStuff v1.6.2, WildSkript v1.8, RandomSK v2.7.1, Skellett v1.7.7e, SharpSK v1.6.0, TuSKe v1.6.7, skRayFall v1.9.7.

I have tried searching the forums.
I have tried a Google search, and found this result. However, it is an unanswered thread.