Overworld settings missing after leaving Paper

Your Minecraft server ran Paper or Purpur, you switched it to Fabric, Forge, NeoForge, Vanilla or a modpack and kept the world, and now it stops at every start. The Console shows these lines, close together:

[main/WARN]: Missing data pack paper
[Worker-Main-1/ERROR]: Unable to read or access the world gen settings file! Falling back to the default settings with a random world seed. ./world/data/minecraft/world_gen_settings.dat
[main/WARN]: Failed to load datapacks, can't proceed with server load. You can either fix your datapacks or reset to vanilla with --safeMode
Caused by: java.lang.IllegalStateException: Overworld settings missing

Your world is not damaged. Paper and Purpur save some of the world's settings (the seed and world generation settings, game rules, weather, the world border) in a different folder from the one the other server software reads, so the new software can't find them. Switching back to Paper and then switching again moves them for you.

What we saw on our own server

We reproduced this on one of our servers on 30 October 2026 with Paper 26.3. Paper keeps those settings in world/dimensions/minecraft/overworld/data/minecraft/, while Fabric 26.3 looks for them in world/data/minecraft/. With the files copied across, the same world started on Fabric in under a second with its original seed, and it started again on Paper afterwards with nothing lost.

The JAR Manager now moves these files by itself when you switch between Paper or Purpur and any other software, and so do modpack installs that keep your world. A server switched before that change needs the steps below once.

The fix

1. Stop the server (Console tab).
2. Take a backup in the Backups tab, so you can go back if anything looks wrong.
3. Open the JAR Manager tab and switch the server back to the software the world was made on (Paper or Purpur), on the same Minecraft version. Click Apply change and wait for the reinstall to finish.
4. Start the server and wait for Done (…s)! in the Console. The world is there as you left it. Stop the server again.
5. In the JAR Manager, switch to the software you wanted (Fabric, Forge, NeoForge or Vanilla) and click Apply change. This time the world's settings are moved before the reinstall.
6. Start the server. Missing data pack paper can still appear as a warning, which is harmless: it only means Paper's own data pack isn't there any more.

If it still won't start

• Your plugins don't come with you. Fabric, Forge and NeoForge run mods, not Paper plugins, so anything a plugin added (land claims, economy balances, permissions) stays in the plugins folder and isn't used.
• Check the version. Minecraft can't open a world saved by a newer version. The JAR Manager refuses that switch, but if the version was changed another way, pick the version the world was last played on, or newer.
• Restore the backup you took in step 2 from the Backups tab if you'd rather go back to how it was.