Steps to reproduce:

1. Create a Java 21w20a server without running it (check server.properties, sign eula)
2. Add a folder for "world_test" add "datapacks" to it and within "world_test/datapacks" add the Caves and Cliffs Preview datapack
3. Run the server (it should load the datapack just fine and try to generate chunks, but before it can it will crash. (check crash report in attachments)

It will work if:

1. Create a Java 21w20a server without running it (check server.properties, sign eula)
2. Run the server (it should NOT load the datapack)
3. Let it generate the chunks and when it's done, close the server
4. Add the Caves and Cliffs Preview datapack to "world_test/datapacks"
5. Restart the server, it works fine now!

Data Pack in Question:
CavesAndCliffsPreview.zip