I was testing out a few changes to my world's data pack. That consisted of increasing the height 512 blocks further down, and adding a new flat world generation overriding the old layers specified when creating the world a decade ago. Every time I made a change, I deleted the world, and pasted in a backup, additionally including the data pack changes. The last change I made, was changing the second layer, stone, from a height (tallness, not offset) of 1 block to 255 blocks. When I started the game and loaded in the world, it pretty much froze. Not exactly. It's running. But very slowly. And uselessly. And spewing lots of errors. See the endlessly long log file generated from this...