I recently downloaded snapshot 20w21a. 

This is a singleplayer map seed: 865409336466754192

This is my first time playing any of the snapshots for the upcoming 1.16 updates and I was excited to explore the world in search of ruined portals. I found one but upon getting even close to the area that spawned my game crashed. I tried again thinking its a fluke and it did this 9 more times. I have not tried any other portal on this world yet but I believe the same thing will happen. 

EDIT: this only happens in survival, I reloaded the world in creative and i can walk around just fine

RECREATE:
Load map on single player with seed: 865409336466754192 in survival
walk to coordinates X: 897 z: 950 (or anywhere around there)

CRASH REPORT
---- Minecraft Crash Report ----
// My bad.

Time: 5/27/20 11:28 PM
Description: Feature placement

java.lang.UnsupportedOperationException
at com.google.common.collect.ImmutableList.sort(ImmutableList.java:565)
at asi.a(SourceFile:43)
at asi.b(SourceFile:60)
at cot.a(SourceFile:42)
at civ.a(SourceFile:56)
at civ.a(SourceFile:35)
at civ.a(SourceFile:17)
at cja.a(SourceFile:44)
at cow.a(SourceFile:82)
at csr.a(SourceFile:83)
at csr.a(SourceFile:79)
at csy.a(SourceFile:75)
at bqd.a(SourceFile:560)
at bqd$$Lambda$4770/1728977309.accept(Unknown Source)
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at it.unimi.dsi.fastutil.longs.LongIterator$$Lambda$4765/186039997.accept(Unknown Source)
at java.util.PrimitiveIterator$OfLong.forEachRemaining(PrimitiveIterator.java:189)
at it.unimi.dsi.fastutil.longs.LongIterator.forEachRemaining(LongIterator.java:53)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
at bqd.a(SourceFile:559)
at cga.a(SourceFile:210)
at cgc.c(SourceFile:84)
at cgc$$Lambda$1707/486970842.doWork(Unknown Source)
at cgc.a(SourceFile:214)
at yi.a(SourceFile:569)
at yi$$Lambda$4051/236855344.apply(Unknown Source)
at com.mojang.datafixers.util.Either$Left.map(Either.java:38)
at yi.a(SourceFile:563)
at yi$$Lambda$4048/856473583.apply(Unknown Source)
at java.util.concurrent.CompletableFuture.uniCompose(CompletableFuture.java:952)
at java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:926)
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442)
at yk.b(SourceFile:58)
at yk$$Lambda$4017/863409293.run(Unknown Source)
at amc.e(SourceFile:94)
at amc.a(SourceFile:137)
at amc.run(SourceFile:105)
at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1689)
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

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

– Head –
Thread: Server thread
Stacktrace:
at com.google.common.collect.ImmutableList.sort(ImmutableList.java:565)
at asi.a(SourceFile:43)
at asi.b(SourceFile:60)
at cot.a(SourceFile:42)
at civ.a(SourceFile:56)
at civ.a(SourceFile:35)
at civ.a(SourceFile:17)
at cja.a(SourceFile:44)
at cow.a(SourceFile:82)
at csr.a(SourceFile:83)
at csr.a(SourceFile:79)
at csy.a(SourceFile:75)
at bqd.a(SourceFile:560)
at bqd$$Lambda$4770/1728977309.accept(Unknown Source)
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at it.unimi.dsi.fastutil.longs.LongIterator$$Lambda$4765/186039997.accept(Unknown Source)
at java.util.PrimitiveIterator$OfLong.forEachRemaining(PrimitiveIterator.java:189)
at it.unimi.dsi.fastutil.longs.LongIterator.forEachRemaining(LongIterator.java:53)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)

– Feature –
Details:
Id: minecraft:village
Description: clq@33e19be9
Stacktrace:
at bqd.a(SourceFile:559)

– Generation –
Details:
CenterX: 70
CenterZ: 65
Step: SURFACE_STRUCTURES
Seed: 3329056681505200320
Biome: minecraft:taiga
Stacktrace:
at cga.a(SourceFile:210)
at cgc.c(SourceFile:84)
at cgc$$Lambda$1707/486970842.doWork(Unknown Source)
at cgc.a(SourceFile:214)

– Chunk to be generated –
Details:
Location: 70,65
Position hash: 279172874310
Generator: cho@5e1245ed
Stacktrace:
at yi.a(SourceFile:569)
at yi$$Lambda$4051/236855344.apply(Unknown Source)
at com.mojang.datafixers.util.Either$Left.map(Either.java:38)
at yi.a(SourceFile:563)
at yi$$Lambda$4048/856473583.apply(Unknown Source)
at java.util.concurrent.CompletableFuture.uniCompose(CompletableFuture.java:952)
at java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:926)
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442)
at yk.b(SourceFile:58)
at yk$$Lambda$4017/863409293.run(Unknown Source)
at amc.e(SourceFile:94)
at amc.a(SourceFile:137)
at amc.run(SourceFile:105)

– Affected level –
Details:
All players: 1 total; [yx['generalmiscreant'/68, l='ServerLevel[snapshot w20a12]', x=897.32, y=80.00, z=933.58]]
Chunk stats: ServerChunkCache: 4416
Level dimension: minecraft:overworld
Level spawn location: World: (96,62,26), Chunk: (at 0,3,10 in 6,1; contains blocks 96,0,16 to 111,255,31), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 153026 game time, 194470 day time
Level name: snapshot w20a12
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Level weather: Rain time: 164459 (now: false), thunder time: 86826 (now: false)
Known server brands: vanilla
Level was modded: false
Level storage version: 0x04ABD - Anvil
Stacktrace:
at net.minecraft.server.MinecraftServer.b(SourceFile:896)
at net.minecraft.server.MinecraftServer.a(SourceFile:832)
at enr.a(SourceFile:89)
at net.minecraft.server.MinecraftServer.run(SourceFile:691)
at java.lang.Thread.run(Thread.java:745)

– System Details –
Details:
Minecraft Version: 20w21a
Minecraft Version ID: 20w21a
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: 1001043136 bytes (954 MB) / 1912602624 bytes (1824 MB) up to 2147483648 bytes (2048 MB)
CPUs: 12
JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
Player Count: 1 / 8; [yx['generalmiscreant'/68, l='ServerLevel[snapshot w20a12]', x=897.32, y=80.00, z=933.58]]
Data Packs: vanilla
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.