Craft Of The Titans server lag

My friend and I got a Craft Of The Titans server but it's extremely laggy!
The more we play the worse it gets.
We don't have any plugins and I checked the console. In the console the is a lot of:"running behind".
The extremley anoying part is that the server crashes a lot!
We (especially I) love this pack so much that we don't want to stop but it's getting hard to not get annoyed by the crashes and then waiting for the server to get past the que and start.
Please help!

Thanks in advance!
appel28

[18:35:57] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 15047ms behind, skipping 300 tick(s)
[18:36:00] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2012ms behind, skipping 40 tick(s)
[18:36:20] [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 net.minecraft.entity.EntityTracker.func_72788_a(EntityTracker.java:280) ~[lp.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:721) ~[MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:387) ~[ld.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:613) ~[MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:471) [MinecraftServer.class:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_144]
[18:36:20] [Server thread/ERROR]: This crash report has been saved to: /aternos/server/./crash-reports/crash-2018-02-18_18.36.20-server.txt
[18:36:20] [Server thread/INFO]: Stopping server

No not really but we shot arrows at each other, shot spells, threw things on the ground and sometimes we really did nothing.