Server crash: Exception loading chunk

See crash report attached.

This crash is triggered whenever the first chunk in the world loads, either by running a forceload command, or by joining the world.
The enabled data pack provides a custom biome and overwrites minecraft:overworld, and that has been fine for years. Disabling ticking/reload functions doesn't fix this.

World folder with data pack reduced to essentials only: https://www.mediafire.com/file/4e38p1miwd1pvmi/MC-277476_-_altered_world_copy.zip/file