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I've found that by removing player data files, loading the world... then closing the world and restoring the same exact playerdatafiles, gets the game to work again till the next crash, which seems to be random.

here is the log file.

---- Minecraft Crash Report ----
// You should try our sister game, Minceraft!

Time: 3/16/18 8:32 PM
Description: Ticking entity

java.lang.ClassCastException: afn cannot be cast to afj
	at adb.a(SourceFile:41)
	at acz.a(SourceFile:22)
	at adl.a(SourceFile:94)
	at abw.cL(SourceFile:644)
	at abv.l(SourceFile:1895)
	at abw.l(SourceFile:504)
	at ajs.l(SourceFile:33)
	at aki.l(SourceFile:257)
	at abv.S_(SourceFile:1750)
	at abw.S_(SourceFile:284)
	at ajs.S_(SourceFile:38)
	at aki.S_(SourceFile:232)
	at ajg.S_(SourceFile:252)
	at aug.a(SourceFile:1617)
	at se.a(SourceFile:705)
	at aug.h(SourceFile:1593)
	at aug.p_(SourceFile:1475)
	at se.p_(SourceFile:538)
	at net.minecraft.server.MinecraftServer.w(SourceFile:702)
	at net.minecraft.server.MinecraftServer.v(SourceFile:627)
	at czd.v(SourceFile:155)
	at net.minecraft.server.MinecraftServer.run(SourceFile:532)
	at java.lang.Thread.run(Thread.java:745)


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

-- Head --
Thread: Server thread
Stacktrace:
	at adb.a(SourceFile:41)
	at acz.a(SourceFile:22)
	at adl.a(SourceFile:94)
	at abw.cL(SourceFile:644)
	at abv.l(SourceFile:1895)
	at abw.l(SourceFile:504)
	at ajs.l(SourceFile:33)
	at aki.l(SourceFile:257)
	at abv.S_(SourceFile:1750)
	at abw.S_(SourceFile:284)
	at ajs.S_(SourceFile:38)
	at aki.S_(SourceFile:232)
	at ajg.S_(SourceFile:252)
	at aug.a(SourceFile:1617)
	at se.a(SourceFile:705)
	at aug.h(SourceFile:1593)

-- Entity being ticked --
Details:
	Entity Type: minecraft:drowned (ajg)
	Entity ID: 318
	Entity Name: Drowned
	Entity's Exact location: -736.20, 40.75, 240.30
	Entity's Block location: World: (-737,40,240), Chunk: (at 15,2,0 in -47,15; contains blocks -752,0,240 to -737,255,255), Region: (-2,0; contains chunks -64,0 to -33,31, blocks -1024,0,0 to -513,255,511)
	Entity's Momentum: 0.00, -0.02, 0.00
	Entity's Passengers: []
	Entity's Vehicle: ~~ERROR~~ NullPointerException: null
Stacktrace:
	at aug.p_(SourceFile:1475)
	at se.p_(SourceFile:538)

-- Affected level --
Details:
	Level name: March2018KIDS
	All players: 1 total; [sg['Typo91'/198, l='March2018KIDS', x=-751.64, y=72.00, z=259.75]]
	Chunk stats: ServerChunkCache: 1994 Drop: 0
	Level seed: 729879487977576903
	Level generator: ID 02 - largeBiomes, ver 0. Features enabled: true
	Level generator options: 
	Level spawn location: World: (-190,64,106), Chunk: (at 2,4,10 in -12,6; contains blocks -192,0,96 to -177,255,111), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 1332474 game time, 1621309 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 154792 (now: false), thunder time: 103177 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.w(SourceFile:702)
	at net.minecraft.server.MinecraftServer.v(SourceFile:627)
	at czd.v(SourceFile:155)
	at net.minecraft.server.MinecraftServer.run(SourceFile:532)
	at java.lang.Thread.run(Thread.java:745)

-- System Details --
Details:
	Minecraft Version: 18w11a
	Operating System: Windows 7 (amd64) version 6.1
	Java Version: 1.8.0_25, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 113182944 bytes (107 MB) / 603979776 bytes (576 MB) up to 8589934592 bytes (8192 MB)
	JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx8G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M
	Profiler Position: N/A (disabled)
	Player Count: 1 / 8; [sg['Typo91'/198, l='March2018KIDS', x=-751.64, y=72.00, z=259.75]]
	Data Packs: vanilla
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.