I received the following crash. It appears to be related to pistons.

[09:50:08] [Server thread/ERROR]: Encountered an unexpected exception
net.minecraft.server.v1_14_R1.ReportedException: Exception ticking world
at net.minecraft.server.v1_14_R1.MinecraftServer.b(MinecraftServer.java:1070) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.DedicatedServer.b(DedicatedServer.java:396) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.MinecraftServer.a(MinecraftServer.java:956) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.MinecraftServer.run(MinecraftServer.java:801) [spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_212]
Caused by: java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445) ~[?:1.8.0_212]
at java.util.HashMap$KeyIterator.next(HashMap.java:1469) ~[?:1.8.0_212]
at net.minecraft.server.v1_14_R1.WorldServer.notify(WorldServer.java:1200) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.World.notifyAndUpdatePhysics(World.java:335) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.World.setTypeAndData(World.java:311) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.BlockPiston.a(BlockPiston.java:205) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.IBlockData.a(SourceFile:238) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.WorldServer.a(WorldServer.java:1276) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.WorldServer.ae(WorldServer.java:1266) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.WorldServer.doTick(WorldServer.java:282) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.MinecraftServer.b(MinecraftServer.java:1057) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
... 4 more
[09:50:08] [Server thread/ERROR]: Cause of unexpected exception was
java.util.ConcurrentModificationException: null
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445) ~[?:1.8.0_212]
at java.util.HashMap$KeyIterator.next(HashMap.java:1469) ~[?:1.8.0_212]
at net.minecraft.server.v1_14_R1.WorldServer.notify(WorldServer.java:1200) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.World.notifyAndUpdatePhysics(World.java:335) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.World.setTypeAndData(World.java:311) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.BlockPiston.a(BlockPiston.java:205) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.IBlockData.a(SourceFile:238) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.WorldServer.a(WorldServer.java:1276) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.WorldServer.ae(WorldServer.java:1266) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.WorldServer.doTick(WorldServer.java:282) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.MinecraftServer.b(MinecraftServer.java:1057) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.DedicatedServer.b(DedicatedServer.java:396) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.MinecraftServer.a(MinecraftServer.java:956) ~[spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at net.minecraft.server.v1_14_R1.MinecraftServer.run(MinecraftServer.java:801) [spigot-1.14.1.jar:git-Spigot-8043ebc-f690957]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_212]
[09:50:08] [Server thread/ERROR]: This crash report has been saved to: /home/minerealm/classic/./crash-reports/crash-2019-05-16_09.50.08-server.txt
[09:50:08] [Server thread/INFO]: Stopping server

Crash report is attached.