Fix No key dimensions in MapLike (level.dat)

Your server stops during boot and the console shows Failed to load datapacks, can't proceed with server load. Look a few lines above it for the real error:

java.lang.IllegalStateException: No key dimensions in MapLike[{}]; No key seed in MapLike[{}]

(It can also appear wrapped as java.util.concurrent.ExecutionException: ... or Caused by: java.lang.RuntimeException: ....) Despite the message, this is not a datapack problem. The world's main save file, level.dat, has lost its world generation settings (WorldGenSettings: the seed and the list of dimensions), so the server can't open the world. Restoring level.dat_old fixes it in a couple of minutes, but read Why it happens too, or the error can come straight back.

What we saw when we broke level.dat on purpose

We damaged level.dat three ways on one of our own servers (Vanilla 1.21.1, Diamond plan, 14 October 2026) to see what Minecraft does with each:

So on a current version, an unreadable level.dat usually fixes itself: Minecraft keeps the bad copy as level.dat_corrupted_ plus the date and time, and you can delete it once the world is back. The case that needs you is the one this guide is named after, where the file reads fine but is missing its settings. The fix below brought our world back on the first start, with Done about 15 seconds after we pressed Start.

Why it happens

• The disk filled up, or the server stopped mid-save. The save was cut short and level.dat was left empty or half written. In our tests Minecraft 1.21.1 recovered from an empty file by itself, using level.dat_old, but a cut-short save can still leave the world unable to load, especially if level.dat_old was hit too.
• The world is missing the mods it was made with. A world that uses world generation or dimension mods saves settings that only those mods can read, so keep it on the software it was made on, with those mods installed.
• A version change broke a modded dimension. Updating a mod that adds a dimension, or moving the world to a different Minecraft version, can leave level.dat pointing at a dimension the server no longer understands.
• A world generation mod fails to save the settings. Some combinations of world generation mods (TerraBlender, Lithostitched, Biolith and mods built on them) hand Minecraft settings it can't write. Minecraft then saves level.dat without them, with no error at the time, and the next boot crashes. Because this happens on every save, restoring the file only lasts until the next save.

The fix

1. Stop the server (Console tab).
2. Open the File Manager and go into the world/ folder.
3. Look for level.dat_old. If it exists and is not 0 bytes, it's an automatic backup of your save file:
   • If you want a spare copy, Download level.dat_old first.
   • Rename level.dat to level.dat.broken.
   • Rename level.dat_old to level.dat.
4. Start the server. When the Console shows Done (…s)!, the world is back.

If there's no good backup

If level.dat_old is missing or 0 bytes too:

1. Restore your latest backup from the Backups tab. How to create and restore backups walks through it. On our test server, a 106 MB backup took about 5 seconds to make and was restored within about 20 seconds.
2. No backups? The world can't be recovered. Delete the world, world_nether and world_the_end folders and start the server to generate a fresh world.

If the error comes back after you restore

The server boots, then crashes with the same error after its next save or restart? Something is writing level.dat broken again, and restoring it a second time won't hold until you find what:

1. Check the disk. The Disk card on the Console tab shows your usage. If you're near the limit, clear old logs and unused world folders first (the Disk Analyser tab shows what's using the space).
2. Match the software and version. In the JAR Manager tab, check the server is on the same software (Vanilla, Paper, Purpur, Fabric, Forge or NeoForge) and Minecraft version the world was made on. If either changed, put it back with Change Minecraft server version or loader, and never move a world to an older version.
3. Check every mod is still there. Compare the mods folder with the mods the world was made with. A missing world generation or dimension mod is enough to cause this. Install Fabric and Forge mods on your server shows how to add it back with the right loader.
4. Update your world generation mods. If you recently added or updated TerraBlender, Lithostitched, Biolith or a mod that depends on them, update each to its newest build for your Minecraft version, restore level.dat_old again and start the server. If it still breaks, report it on that mod's issue tracker with your crash log.
5. Restore from before the change. In the Backups tab, restore a backup taken before you added the mod or changed version.

Stop it happening again

On servers without mods, a full disk is the likeliest way both copies get damaged. Keep an eye on the Disk card on the Console tab, and when usage nears your plan's limit, clear old logs and unused world folders or upgrade your plan.