the server goes offline without reason, why?

I made a server with only the software mod call of battle, problem, after some time, my friend and I are kicked from the server, which then goes offline for no apparent reason, help me plz T-T

I looked in the console and log, but I did not see any error message
if I remember correctly, on my screen, when I get kicked from the server, the message just tells me: "the server is close"

Aaaah excuse me I looked very badly! already the message on my screen is "server closed"
then a crash reports has been recorded, the console tells me:
"[12:28:22] [Server thread/ERROR]: Encountered an unexpected exception
java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1437) ~[?:1.8.0_144]
at java.util.HashMap$KeyIterator.next(HashMap.java:1461) ~[?:1.8.0_144]
at ww2.WWIIServerTicker.sTick(WWIIServerTicker.java:1064) ~[WWIIServerTicker.class:?]
at ww2.WWIIServerTicker.onServerTick(WWIIServerTicker.java:289) ~[WWIIServerTicker.class:?]
at cpw.mods.fml.common.eventhandler.ASMEventHandler_6_WWIIServerTicker_onServerTick_ServerTickEvent.invoke(.dynamic) ~[?:?]
at cpw.mods.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:54) ~[ASMEventHandler.class:?]
at cpw.mods.fml.common.eventhandler.EventBus.post(EventBus.java:140) ~[EventBus.class:?]
at cpw.mods.fml.common.FMLCommonHandler.onPostServerTick(FMLCommonHandler.java:247) ~[FMLCommonHandler.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:590) ~[MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685) [li.class:?]"

and "[12:28:22] [Server thread / ERROR]: This crash has been saved to: /aternos/server/./crash-reports/crash- 2017-12-02_12.28.22-server.txt"