ReportedException: Saving entity NBT (Invalid Item Data in 1.20.5+)

What the log looks like

net\.minecraft\.ReportedException: Saving entity NBT
Failed to save entity data \(-?\d+,-?\d+\) in world

How to fix it

1. Check the `Caused by: java.lang.IllegalStateException` line at the bottom of the stack trace to identify the exact rule violation (such as `Value must be within range [1;99]` or `Player name contained disallowed characters`).
2. If the error mentions range `[1;99]` on a Paper server, open `config/paper-world-defaults.yml` (or your per-world config) and ensure `fixes.split-overstacked-loot` is set to `true` so high-level Fortune drops split into legal stacks.
3. Note the chunk coordinates in the `Failed to save entity data (X,Z)` log message, multiply X and Z by 16 to find the block coordinates, and clear dropped items or armor stands there (e.g., `/kill @e[type=item,distance=..50]`).
4. If the error mentions `Player name contained disallowed characters`, update or reconfigure the plugin, datapack, or command generating custom player skulls so profile names only use valid characters (letters, numbers, and underscores, max 16 characters).
5. Audit plugins, custom enchantments, and creative-mode items to ensure nothing spawns item stacks exceeding a count of 99.

Check your own log