There is a datapack incompatibility between the latest version of Occultism and JAOPCA. I reported it both here and on the other mod's issue tracker. When creating or loading a world the "Datapack loading error" screen appears. I've narrowed it down to these two mods.

• Mincraft Version: 1.16.5
• Forge Version: 36.1.61
• Occultism Version: 1.7.0
• JOAPCA Version: 3.4.0.9

Should be fixed in 3.4.0.10

A user posted a crashlog on the occultism GitHub repo: https://gist.github.com/Febilian/3d86f120b1935ebd1336f229b675343e

See also klikli-dev/occultism#306

My best guess is that the recent change in Occultism 1.7.0 that added the property ignore_crushing_multiplier to crushing recipes causes this - based on the log my best guess is the compat layer needs to be updated.

See also the related commit on occultism klikli-dev/occultism@e41c086