The Bug

An error of this type occurs when the provided datapack fails to update experimental worldgen settings during a second world load leaving that world entirely inaccessible even when datapacks are removed from their directory.

Workaround

A possible fix would be to remove the affected worldgen settings from the level.dat (using a tag editor like NBT Explorer).

Although, as I'm not completely sure what causes this issue to occur, it seems that the custom dimension settings used to generate the new dimension are unable to be deleted if the game cannot locate those files in the datapacks (according to the game log).

Reproduction steps:

• Create a world with the attached experimental datapack
• Make sure the datapack is enabled in the world
• Disable the datapack providing the experimental settings
• Quit the game
• Attempt to enter the world again