Bukkit

What steps will reproduce the problem?
1. Create a new server, based on Spigot 1.8 (built on 2015-02-14).
2. Install TNT Tag, create an arena, register 3 players or more for a match.
3. Once the first round's countdown hits 0, tagged players are ejected, new ones are named, but the countdown never resets for a new round.

What is the expected output? What do you see instead?
I'd have expected a new round to begin with the remaining players.

What version of the product are you using?
TNT Tag v1.2.1

Do you have an error log of what happened?
This is the log output at the time of the problem:
[23:16:15] [Server thread/WARN]: [TNTTag] Task #9 for TNTTag v1.2.1 generated an exception
java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901) ~[?:1.8.0_31]
    at java.util.ArrayList$Itr.next(ArrayList.java:851) ~[?:1.8.0_31]
    at org.pato.tnttag.managers.CountdownManager.blowUpTNTs(CountdownManager.java:198) ~[?:?]
    at org.pato.tnttag.managers.CountdownManager$2.run(CountdownManager.java:149) ~[?:?]
    at org.bukkit.craftbukkit.v1_8_R1.scheduler.CraftTask.run(CraftTask.java:71) ~[spigot-1.8.jar:git-Spigot-4faf77a-17a3db7]
    at org.bukkit.craftbukkit.v1_8_R1.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:350) [spigot-1.8.jar:git-Spigot-4faf77a-17a3db7]
    at net.minecraft.server.v1_8_R1.MinecraftServer.z(MinecraftServer.java:709) [spigot-1.8.jar:git-Spigot-4faf77a-17a3db7]
    at net.minecraft.server.v1_8_R1.DedicatedServer.z(DedicatedServer.java:316) [spigot-1.8.jar:git-Spigot-4faf77a-17a3db7]
    at net.minecraft.server.v1_8_R1.MinecraftServer.y(MinecraftServer.java:634) [spigot-1.8.jar:git-Spigot-4faf77a-17a3db7]
    at net.minecraft.server.v1_8_R1.MinecraftServer.run(MinecraftServer.java:537) [spigot-1.8.jar:git-Spigot-4faf77a-17a3db7]
    at java.lang.Thread.run(Thread.java:745) [?:1.8.0_31]

Please provide any additional information below.
It works fine for 2 players only, but breaks at 3+.