As-tu vérifié ta console (ton invite de commande) pour voir si cela n'a pas craché ?
Peut être que c'est un problème de version de multiverse-core, essaye de le réinstaller par bukkit, fichiers, et tu choisis la version de ton serveur.
Cordialement JLEUS
[14:50:31] [Server thread/ERROR] [minecraft/EntityTracker]: "Silently" catching entity tracking error.
net.minecraft.util.ReportedException: Adding entity to track
at net.minecraft.entity.EntityTracker.func_72785_a(EntityTracker.java:182) [ol.class:?]
at net.minecraft.entity.EntityTracker.func_72786_a(EntityTracker.java:113) [ol.class:?]
at net.minecraft.world.ServerWorldEventHandler.func_72703_a(ServerWorldEventHandler.java:33) [op.class:?]
at net.minecraft.world.World.func_72923_a(World.java:1165) [amu.class:?]
at net.minecraft.world.WorldServer.func_72923_a(WorldServer.java:1246) [oo.class:?]
at net.minecraft.world.World.spawnEntity(World.java:1158) [amu.class:?]
at net.minecraft.world.WorldServer.spawnEntity(WorldServer.java:1205) [oo.class:?]
at net.minecraft.world.World.func_72838_d(World.java:1098) [amu.class:?]
at net.minecraft.world.WorldServer.func_72838_d(WorldServer.java:1199) [oo.class:?]
at net.minecraft.entity.EntityLivingBase.func_70645_a(EntityLivingBase.java:1135) [vp.class:?]
at net.minecraft.entity.EntityLivingBase.func_70097_a(EntityLivingBase.java:972) [vp.class:?]
at net.minecraft.entity.EntityLivingBase.func_70030_z(EntityLivingBase.java:263) [vp.class:?]
at net.minecraft.entity.EntityLiving.func_70030_z(EntityLiving.java:231) [vq.class:?]
at net.minecraft.entity.Entity.func_70071_h_(Entity.java:438) [vg.class:?]
at net.minecraft.entity.EntityLivingBase.func_70071_h_(EntityLivingBase.java:1998) [vp.class:?]
at net.minecraft.entity.EntityLiving.func_70071_h_(EntityLiving.java:297) [vq.class:?]
at net.minecraft.world.World.func_72866_a(World.java:1853) [amu.class:?]
at net.minecraft.world.World.func_72870_g(World.java:1821) [amu.class:?]
at net.minecraft.world.World.func_72939_s(World.java:1637) [amu.class:?]
at net.minecraft.world.WorldServer.func_72939_s(WorldServer.java:753) [oo.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:733) [MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:385) [nz.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:614) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:505) [MinecraftServer.class:?]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_231]
Caused by: java.lang.IllegalStateException: Entity is already tracked!
at net.minecraft.entity.EntityTracker.func_72785_a(EntityTracker.java:156) ~[ol.class:?]
... 24 more
Voici la console