Minecraft Server Broke After a Modpack or Software Change?

If your server broke after a modpack install, a version change, a software switch, or a new mod, and FORMAT didn’t run, the server wasn’t wiped. What broke is usually the match between your files and your server software.

Before you change anything

1. Stop the server. Don’t edit files while it’s online.
2. Archive everything in the root directory, even if it’s broken, and download the archive. See Back Up Your Server Files Manually.
3. Leave FORMAT off until your archive is downloaded, and don’t repeat installs. FORMAT in Install Different Edition and in the Modpack Manager permanently deletes your world, configs, plugins, and mods. A few fixes below use FORMAT on purpose, and each one assumes you have the archive from step 2.

Step 1: Check Java and the server jar

In the Game Control Panel sidebar, click Server Options and check two settings:

• Java Version must match your Minecraft version. The wrong one stops the server from starting. Look yours up in Which version do I need?, then restart.
• Server Jar File is managed by the panel and may show mc-wrapper.jar. Leave it as the panel set it.

If Server Jar File was changed and the server no longer starts, don’t guess at a new value. Reinstall your software under Advanced > Server Actions > Install Different Edition with FORMAT off (see How to Use the Minecraft Edition Installer), or ask our support team to set it back.

Step 2: Read the error

Open the Console, or logs/latest.log in the File Manager, and find the first FATAL or ERROR line.

What you see	Go to
Unsupported class file major version	Step 1
Unable to access jarfile, or the server starts as plain Vanilla and ignores mods	Step 1, then You switched server software
for invalid dist DEDICATED_SERVER, MissingModsException, or “requires” followed by a mod name	You added or removed a mod
Forge or NeoForge loads zero mods	You uploaded a pack by hand
The world won’t load after a software or version switch	You switched server software or You changed the Minecraft version
Players are disconnected on join or see a mismatched-mods screen	Can’t connect after installing
Install Failed in the panel	”Install Failed” When Opening Your Server
Exit code 137, or a crash notice saying the server ran out of memory	Out of Memory (OOM) Crashes
No error that points to your change	Server Won’t Start or Keeps Crashing?

Step 3: Fix what you changed

A. You installed or switched to a different modpack

Planning a switch? Follow Switch to a different modpack. If it already broke:

• You installed over the old pack. Archive everything, delete everything in the root except the archive, then install the pack again from the Modpack Manager. Leave FORMAT off, since it would delete your archive too, and click Install.
• Reinstalling the same pack still fails. Download your archive, then reinstall the pack from the Modpack Manager with FORMAT on for a clean install. FORMAT deletes every file on the server first, the archive included, so keep your downloaded copy.
• The old world won’t load under the new pack. Keep it in your archive, rename the world folder (for example to world.old), and start the server to make a fresh world.
• The pack isn’t listed or won’t install. See Install a Modpack Manually.

Prefer we handle it? Message us on Discord or open a Client Area ticket with the modpack and version, or attach your own pack export, and our support team will install it for free. We’ll keep your current world unless you ask for a fresh one.

B. You updated your modpack

Start with Server won’t start after updating? Two more cases:

• Mods you added by hand are gone. Deleting the mods folder removes them. Pull them from your archive and add back only the ones that match the new pack’s Minecraft version and loader.
• Players can’t join. Everyone needs the same pack version as the server.

C. You switched server software

Install Different Edition, under Advanced > Server Actions, has two choices: EDITION (the software) and VERSION (the Minecraft version). A wrong pick in either gives you a different server than the one your world was built on.

• Mods on plugin software, or plugins on a modloader. Mods run on Forge, NeoForge, and Fabric. Plugins run on Paper, Purpur, and Spigot. See Server Software Types.
• The world won’t load. Paper, Spigot, and Vanilla-based software store the Nether and End in different places. See Spigot & Vanilla World Formats.
• You switched between modloaders. Their mods don’t load on each other, and leftover files cause startup errors. Archive your world, then do a clean install with FORMAT on.

To undo the switch, reinstall the same edition and the same version you ran before, with FORMAT off, then check the Java Version. Not sure what you ran? The first lines of logs/latest.log show it. See Checking Your Server Version.

D. You changed the Minecraft version

A world that has opened on a newer version can’t go back. See Updating Minecraft Java Edition.

• The world won’t load. Reinstall your previous version with FORMAT off, then restore the archive you took before the change.
• Mods or plugins fail. Each one has to support your new Minecraft version and your loader. Update or remove the ones that don’t.

E. You added or removed a mod or plugin

• for invalid dist DEDICATED_SERVER: a client-only mod is on the server. See Client-side mods crashing the server.
• A missing dependency: see A Mod Is Missing a Dependency.
• Wrong loader or version: a Fabric mod won’t load on Forge, and a 1.20.1 mod won’t load on 1.21.

Add mods in small batches, with an archive between batches. Can’t find the bad one? See Find Which Mod or Plugin Is Causing Problems.

F. You uploaded a pack by hand

• Mods are .jar files, not a .zip. A zip sitting in mods/ loads zero mods. Extract it on your PC and upload the .jar files.
• No extra mods/mods folder. The jars have to sit directly in mods/.
• No libraries folder or loader jar from the pack. Installing your modloader with Install Different Edition already created those.
• Use the pack’s Server Pack if it has one. A CurseForge export holds a manifest.json plus whichever files were selected, so it may not run as a server.

Full steps are in Install a Modpack Manually. Rather not upload it by hand? Send your pack export on Discord or in a Client Area ticket, and our support team will install it for free.

Get back to a working server

Work down this list and stop at the first step that works:

1. Undo the change. Reinstall your previous edition and version with FORMAT off, then set the Java Version.
2. Restore your archive. See Switch back later.
3. Ask us to restore a WinterNode snapshot. Message us on Discord or open a Client Area ticket and ask for the snapshot from just before your change. Our support team will restore it and, if you ask, put the files back in place for you. Replies average about an hour. See Restoring from a WinterNode snapshot.
4. Keep the world, rebuild the rest. Download your world folder, do a clean install with FORMAT on, then import the world or ask our support team to import it for you. Do this only once you’ve confirmed the new setup supports that world.

Still Not Working?

Contact our support team on Discord or in a Client Area ticket with:

• Your Server UUID
• What you changed, and roughly when (so we can match a backup)
• The modpack name and exact version, or the edition and Minecraft version
• The last 50 lines of the console
• Whether you archived the server before the change

If you’d rather we put the modpack back on for you, say so in your message. We install Modpack Manager packs and your own pack exports for free.

While you wait, don’t keep re-running installs or Format. Repeating an install can overwrite files we could otherwise recover.