Bukkit

What is the expected output? What do you see instead?
Trains should be teleportet to an other world (nether), but instead, i got this error and the carts stay.
the sign links trough the "my world" plugin does work, as if i put a portal block close to the sign, it ports me forth and back, aslong as i dont use carts/send carts.

What version of the product are you using?
TrainCarts-1.73.0.jar

Do you have an error log of what happened?
[23:27:07 ERROR]: [Train Carts] Failed to execute GROUP_ENTER for SignActionTeleport:
java.util.ConcurrentModificationException
        at net.minecraft.server.v1_7_R1.World$1.guard(World.java:48) ~[spigot-1.7.2-R0.3-SNAPSHOT.jar:git-Spigot-1223]
        at net.minecraft.server.v1_7_R1.World$1.remove(World.java:42) ~[spigot-1.7.2-R0.3-SNAPSHOT.jar:git-Spigot-1223]
        at net.minecraft.server.v1_7_R1.World.removeEntity(World.java:1049) ~[spigot-1.7.2-R0.3-SNAPSHOT.jar:git-Spigot-1223]
        at com.bergerkiller.bukkit.common.entity.CommonEntity.teleport(CommonEntity.java:460) ~[?:?]
        at com.bergerkiller.bukkit.common.bases.ExtendedEntity.teleport(ExtendedEntity.java:712) ~[?:?]
        at com.bergerkiller.bukkit.tc.controller.MinecartGroup.teleportMember(MinecartGroup.java:463) ~[?:?]
        at com.bergerkiller.bukkit.tc.controller.MinecartGroup.teleport(MinecartGroup.java:452) ~[?:?]
        at com.bergerkiller.bukkit.tc.controller.MinecartGroup.teleport(MinecartGroup.java:437) ~[?:?]
        at com.bergerkiller.bukkit.tc.controller.MinecartGroup.teleport(MinecartGroup.java:434) ~[?:?]
        at com.bergerkiller.bukkit.tc.controller.MinecartGroup.teleportAndGo(MinecartGroup.java:425) ~[?:?]
        at com.bergerkiller.bukkit.tc.signactions.SignActionTeleport.execute(SignActionTeleport.java:57) ~[?:?]
        at com.bergerkiller.bukkit.tc.signactions.SignAction.executeAll(SignAction.java:243) ~[?:?]
[23:27:07 INFO]: Stopping server

I have a newer version of this error as of April. Teleporting was working briefly, but broke after an update. I receive no errors. I have filed a separate ticket for my error, since it is not identical.