Creating a server with the Terralith datapack in a server with a superflat world loaded will, upon joining or teleporting to the world, crash the client and spam exceptions like:

java.lang.RuntimeException: Unknown registry element in ResourceKey[minecraft:root / minecraft:worldgen/biome]:net.minecraft.world.level.biome.BiomeBase@2acec436

I have confirmed the datapack works in a vanilla server.

The server was set up as follows:

1. A singleplayer world was created with the datapack
2. The server was started the first time to generate the necessary files and then stopped
3. The world, world_nether and world_the_end folders were removed
4. The singleplayer folder world was then transferred to the server and its parent folder renamed to "world"
5. There are a few different ways to reproduce the next steps:
   1. Use the /mv create superflat normal -t flat  or create a superflat world in singleplayer, bring it over to the server and use /mv import. The third way is to create a simple plugin with a WorldCreator that will create or load a superflat world, instead of using Multiverse for that. It may be necessary to teleport to the world, log out and in or stop the server to see the errors or experience crashes

I could not reproduce without a main world with a datapack.

Client crash and server logs are attached.