I don't know what I did wrong! One minute it works, the next minute the client crashes when I join a specific world. I tried doing it again, and again, and again. Yet it still crashes!

Crash:

---- Minecraft Crash Report ----
// Ouch. That hurt

Time: 11/29/18 8:52 PM
Description: Exception in server tick loop

java.util.concurrent.CompletionException: java.lang.AbstractMethodError: bxx$$Lambda$1102/2031373582.a(Lcom/mojang/datafixers/Dynamic;)Ljava/lang/Object;
at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1584)
at ui$$Lambda$2134/866962769.accept(Unknown Source)
at java.util.ArrayList.forEach(ArrayList.java:1249)
at ui.e(SourceFile:369)
at net.minecraft.server.MinecraftServer.q(SourceFile:641)
at net.minecraft.server.MinecraftServer.p(SourceFile:625)
at net.minecraft.server.MinecraftServer.a(SourceFile:422)
at dog.a(SourceFile:91)
at dog.d(SourceFile:107)
at net.minecraft.server.MinecraftServer.run(SourceFile:562)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.AbstractMethodError: bxx$$Lambda$1102/2031373582.a(Lcom/mojang/datafixers/Dynamic;)Ljava/lang/Object;
at yg.a(SourceFile:18)
at cat.<init>(SourceFile:44)
at bvl$a.<init>(SourceFile:29)
at bxc$$Lambda$1040/1502502072.load(Unknown Source)
at cax.a(SourceFile:86)
at bqu.a(SourceFile:614)
at bqu.a(SourceFile:229)
at ui.d(SourceFile:439)
at ui$$Lambda$2116/461544538.get(Unknown Source)
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1582)
... 10 more

A detailed walkthrough of the error, its code path and all known details is as follows:
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– System Details –
Details:
Minecraft Version: 18w48a
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_51, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 195234808 bytes (186 MB) / 922746880 bytes (880 MB) up to 4294967296 bytes (4096 MB)
JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx4G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M
Player Count: 0 / 8; []
Data Packs: vanilla
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.