I'm not really sure what's going on here, but somehow the dragon is making the server crash.

Seems to be because EnderDragonBattle believes that there are -3 ender crystals alive (from field h in EnderDragonBattle)
The only other call to random with a variable bound is with an absolute value + 2, so that can't be negative unless something is really wrong.

Actually, h appears to be the total amount of crystals and is never decremented.
Either the integer cache got corrupted somehow, or something modifies the field because h is either 0 or the returned value of Collection#size (so always >=0).

Here is the full crash report:
[16:17:06] [Server thread/WARN]: java.lang.IllegalArgumentException: bound must be positive [16:17:06] [Server thread/WARN]: at java.base/java.util.Random.nextInt(Random.java:388) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.DragonControllerHold.j(SourceFile:70) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.DragonControllerHold.c(SourceFile:37) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.EntityEnderDragon.movementTick(EntityEnderDragon.java:180) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.EntityLiving.tick(EntityLiving.java:2396) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.EntityInsentient.tick(EntityInsentient.java:288) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.WorldServer.entityJoinedWorld(WorldServer.java:807) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.World.a(World.java:879) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.WorldServer.doTick(WorldServer.java:532) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.MinecraftServer.b(MinecraftServer.java:1319) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.DedicatedServer.b(DedicatedServer.java:430) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.MinecraftServer.a(MinecraftServer.java:1173) [16:17:06] [Server thread/WARN]: at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:962) [16:17:06] [Server thread/WARN]: at java.base/java.lang.Thread.run(Thread.java:835)