When trying to load a specific world I have that worked in versions 21w19a and before, I get an error about not being able to read the "u" field.

In single player, it says that it's because "☃" is null, while on multiplayer it says it's because "☃" is null. (Snowman emoji

I saw in the 1.17 Pre Release 1 notes that MC-226192 was fixed and looked very much like this, but sadly this instance of the NullPointerException still happens 1.17-pre1.

I could upload the world file if need be, but the relevant thing to know is that it uses the data pack based custom dimensions features.