IllegalStateException: Invalid pool element found (minecraft:jigsaw)

What the log looks like

java\.lang\.IllegalStateException: Invalid pool element found
Exception loading structure piece with id minecraft:jigsaw[\s\S]{0,200}Invalid pool element found

How to fix it

1. Back up your world folders before proceeding, especially if you are in the middle of a server version upgrade.
2. If you recently removed a worldgen datapack or structure mod, restore it to your world's datapacks (or mods) folder and restart so the server can recognize the saved pool elements.
3. If you are running Paper (especially early 1.18 or 1.18.1 builds like #60–#77), update to the latest build for your Minecraft version to fix async registry and datafixer errors during chunk loading.
4. Start your server once with the --forceUpgrade flag placed after the server jar name in your startup script; this converts and re-saves all chunks, clearing out the unreadable legacy jigsaw tags so console spam stops.
5. If importing an older world with Multiverse-Core causes the server to stall while logging these errors, upgrade that world first on a clean server instance using --forceUpgrade before running /mv import.

Check your own log