Server crashing when trying to load a pre-existing world into it

Getting this crash report when trying to load a single player world into it. Furthermore, I doesn't seem to run a pre-existing multiplayer world in 20w16a. Any ideas why? I'm running the server on ubuntu 18.04 with java 1.8.0_242.

Crash message (for the 1.15.2 world - the 20w16a world does crash but doesn't provide a crash report at /opt/minecraft/server/./crash-reports):

---- Minecraft Crash Report ----
// I blame Dinnerbone.
Time: 4/20/20 11:11 PM
Description: Exception in server tick loop
java.lang.RuntimeException: Failed to check session lock, aborting
 at crn.h(SourceFile:95)
 at crn.<init>(SourceFile:51)
 at crp.a(SourceFile:113)
 at crp.a(SourceFile:109)
 at net.minecraft.server.MinecraftServer.a(SourceFile:360)
 at wd.d(SourceFile:219)
 at net.minecraft.server.MinecraftServer.run(SourceFile:628)
 at java.lang.Thread.run(Thread.java:748)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------
-- System Details --
Details:
 Minecraft Version: 1.15.2
 Minecraft Version ID: 1.15.2
 Operating System: Linux (amd64) version 4.15.0-96-generic
 Java Version: 1.8.0_242, Private Build
 Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), Private Build
 Memory: 389079584 bytes (371 MB) / 752353280 bytes (717 MB) up to 954728448 bytes (910 MB)
 CPUs: 4
 JVM Flags: 2 total; -Xmx1024M -Xms512M
 Player Count: 0 / 20; []
 Data Packs:
 Is Modded: Unknown (can't tell)
 Type: Dedicated Server (map_server.txt)

So to summarize:

1. My server can't load a 20w16a pre-existing world, even though the server.jar version is the one mojang is posting in the snapshot page over at minecraft.net

2. When switching to 1.15.2, this crash report is outputted to /opt/minecraft/server/./crash-reports 

3. When the 20w16a server crash occurs, no crash report is outputted, only this message in /logs/latest.log:

[13:25:31] [main/ERROR]: Unable to bootstrap registry 'minecraft:chunk_generator_type'
[13:25:31] [main/ERROR]: Registry 'minecraft:chunk_generator_type' was empty after loading
[13:25:31] [main/INFO]: Environment: authHost='https://authserver.mojang.com', accountsHost='https://api.mojang.com', sessionHost='https://sessionserver.mojan...', n$
[13:25:32] [main/FATAL]: Failed to start the minecraft server
java.nio.file.AccessDeniedException: ./world/session.lock
at sun.nio.fs.UnixException.translateToIOException(UnixException.java:84) ~[?:1.8.0_242]
at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102) ~[?:1.8.0_242]
at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107) ~[?:1.8.0_242]
at sun.nio.fs.UnixFileSystemProvider.newFileChannel(UnixFileSystemProvider.java:177) ~[?:1.8.0_242]
at java.nio.channels.FileChann...(FileChannel.java:287) ~[?:1.8.0_242]
at java.nio.channels.FileChann...(FileChannel.java:335) ~[?:1.8.0_242]
at act.a(SourceFile:25) ~[server.jar:?]
at cyw$a.<init>(SourceFile:196) ~[server.jar:?]
at cyw.c(SourceFile:185) ~[server.jar:?]
at net.minecraft.server.MinecraftServer.main(SourceFile:940) [server.jar:?]