Random crashes, log says 'ticking entity' and mentions 'Drowned' mob.

Tried attaching the report, but the forum gives me an error so here it is below:

---- Minecraft Crash Report ----
// Would you like a cupcake?

Time: 3/16/18 11:41 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: 244
Entity Name: Drowned
Entity's Exact location: -614.30, 40.27, -83.81
Entity's Block location: World: (-615,40,-84), Chunk: (at 9,2,12 in -39,-6; contains blocks -624,0,-96 to -609,255,-81), Region: (-2,-1; contains chunks -64,-32 to -33,-1, blocks -1024,0,-512 to -513,255,-1)
Entity's Momentum: 0.00, 0.07, 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: Snapshot w8a09?
All players: 1 total; [sg['BillyOatmeal'/20, l='Snapshot w8a09?', x=-577.43, y=11.00, z=-128.02]]
Chunk stats: ServerChunkCache: 881 Drop: 0
Level seed: -940041908687425
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: World: (-228,64,-84), Chunk: (at 12,4,12 in -15,-6; contains blocks -240,0,-96 to -225,255,-81), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
Level time: 1577013 game time, 2125909 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 152407 (now: false), thunder time: 18135 (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 10 (amd64) version 10.0
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 331190128 bytes (315 MB) / 603979776 bytes (576 MB) up to 1073741824 bytes (1024 MB)
JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -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['BillyOatmeal'/20, l='Snapshot w8a09?', x=-577.43, y=11.00, z=-128.02]]
Data Packs: vanilla
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.