Not much to add here. Server crashed as I was walking in new lands. Had taken the nether to about 8000 normal level blocks away from spawn (map created with an older version–1.12.1 I think) and was heading back via the normal level, so it could be this happened where the "new lands" met what had already been generated.

Crash report is copy/pasted below and attached. Would attach the world also, but it's about 0.5 GB so will wait till requested.

---- Minecraft Crash Report ----
// Sorry

Time: 6/22/18 4:38 PM
Description: Exception generating new chunk

java.util.concurrent.ExecutionException: java.lang.ClassCastException: bfv cannot be cast to bhi
at java.util.concurrent.CompletableFuture.reportGet(Unknown Source)
at java.util.concurrent.CompletableFuture.get(Unknown Source)
at so.c(SourceFile:126)
at sw.a(SourceFile:85)
at sx.c(SourceFile:143)
at sp.i_(SourceFile:222)
at net.minecraft.server.MinecraftServer.w(SourceFile:693)
at sa.w(SourceFile:365)
at net.minecraft.server.MinecraftServer.v(SourceFile:626)
at net.minecraft.server.MinecraftServer.run(SourceFile:531)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassCastException: bfv cannot be cast to bhi
at cav$c.a(SourceFile:1884)
at caq.a(SourceFile:60)
at bwy.a(SourceFile:77)
at bze.a(SourceFile:16)
at bze.a(SourceFile:13)
at bta.a(SourceFile:27)
at bcl.a(SourceFile:515)
at bqm.a(SourceFile:109)
at td.a(SourceFile:12)
at tc.a(SourceFile:35)
at bqq.a(SourceFile:95)
at tj.a(SourceFile:61)
at tj.a(SourceFile:24)
at agt$a.a(SourceFile:131)
at java.util.concurrent.CompletableFuture.uniApply(Unknown Source)
at java.util.concurrent.CompletableFuture$UniApply.tryFire(Unknown Source)
at java.util.concurrent.CompletableFuture$Completion.exec(Unknown Source)
at java.util.concurrent.ForkJoinTask.doExec(Unknown Source)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(Unknown Source)
at java.util.concurrent.ForkJoinPool.runWorker(Unknown Source)
at java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– Head –
Thread: Server thread
Stacktrace:
at java.util.concurrent.CompletableFuture.reportGet(Unknown Source)

– Chunk to be generated –
Details:
Location: 269,-24
Position hash: -103079214835
Generator: bsi@70b750c7
Stacktrace:
at so.a(SourceFile:170)
at so.c(SourceFile:129)
at sw.a(SourceFile:85)
at sx.c(SourceFile:143)
at sp.i_(SourceFile:222)
at net.minecraft.server.MinecraftServer.w(SourceFile:693)
at sa.w(SourceFile:365)
at net.minecraft.server.MinecraftServer.v(SourceFile:626)
at net.minecraft.server.MinecraftServer.run(SourceFile:531)
at java.lang.Thread.run(Unknown Source)

– Affected level –
Details:
Level name: world
All players: 1 total; [sr['Throthlethroth'/236, l='world', x=4831.16, y=71.00, z=117.67]]
Chunk stats: ServerChunkCache: 4846 Drop: 0
Level seed: -7767047874017935925
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options: null
Level spawn location: World: (224,64,256), Chunk: (at 0,4,0 in 14,16; contains blocks 224,0,256 to 239,255,271), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 465954466 game time, 467360962 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 96821 (now: false), thunder time: 88407 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false

– System Details –
Details:
Minecraft Version: 1.13-pre3
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_171, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 5729566528 bytes (5464 MB) / 8507097088 bytes (8113 MB) up to 8507097088 bytes (8113 MB)
JVM Flags: 2 total; -Xmx8192M -Xms8192M
Profiler Position: N/A (disabled)
Player Count: 1 / 20; [sr['Throthlethroth'/236, l='world', x=4831.16, y=71.00, z=117.67]]
Data Packs: vanilla
Is Modded: Unknown (can't tell)
Type: Dedicated Server (map_server.txt)