The 1.20.12 hotfix just popped up claiming to fix the "There was a problem loading this world" bug which has been preventing many players from entering  their worlds.

It's easy to spot cause the game stucks on a "Loading resource packs" screen for minutes before either popping up the message or glitching the screen and then crashing.

Experimental worlds are prone to crash, but it happens in other regular worlds. Big worlds also seem more prone to suffer from this. Please fix it, there's a lot of work on our worlds to leave behind just for a bug.