Modpack won't start or freezes on boot

When a modded server hangs partway through startup — the console goes quiet and never reaches "Done" — or crashes on boot, it's almost always a mod conflict or a broken/outdated mod, not a problem with the server itself. Here's how to find the culprit and get running again.

Frozen vs crashed — tell them apart

• Crashed: the console prints an error and the process stops. You'll see a stack trace and often a crash report under crash-reports/.
• Frozen (hung): the server looks "up" but never finishes loading and no one can join. The log just stops mid-init. This usually means two mods are deadlocking each other during startup.

Both are diagnosed the same way: read the log and isolate the offending mod.

1. Read the log, top to bottom

Open the Console, restart, and read from the top. For Forge/NeoForge the most useful file is logs/fml-server-latest.log (open it in the File Manager). Search for the first occurrence of:

ERROR   FATAL   Caused by:   LoadingFailedException   NoClassDefFound

The first mod that errors is usually the root cause. Mods listed afterward as "errored because dependency X errored" are just downstream noise — ignore them and focus on the first one.

2. Watch for fighting performance mods

Performance mods (Redirector, Canary, ModernFix, Rubidium, Embeddium, and similar) patch the game's internals deeply, and outdated versions can deadlock each other on boot — the classic "server looks up but is frozen mid-init" symptom. If your pack froze after an update, or you're running mod versions that are a few months old, these are prime suspects.

3. Disable the suspect mod

You don't have to delete anything — just take it out of the load path:

1. In the File Manager, open the mods/ folder.
2. Rename the suspect jar from something.jar to something.jar.disabled. (The .disabled extension stops it loading and makes it trivial to re-enable later.)
3. Restart and watch the console.

If it now boots past the freeze/crash point, you found it. If not, re-enable it and try the next suspect. Change one mod at a time so you know which one mattered.

4. Protect your world before a fresh start

If a world became corrupted (or you want to test a clean boot), don't delete it — archive it:

1. In the File Manager, rename world to world-broken-<date>.old.
2. Restart. The server generates a fresh world and boots cleanly.
3. Your old world is still there if you want it back later.

Heads up — world generation is slow with big packs. After a fresh start, a heavy modpack can spend 5–15 minutes deep in chunk generation before it reaches "Done" — mods like MineColonies or Alex's Mobs scanning biomes at boot is normal. If CPU is doing real work (not pinned at 0%), give it time before assuming it's stuck.

5. Fix it properly: update the mods

Disabling a performance mod costs you some FPS but nothing gameplay-breaking, so it's a fine stop-gap. The real fix is usually to update the conflicting mods — most have had bug-fix releases that resolve these conflicts. Grab the newer versions for your Minecraft/loader version, drop them into mods/ (replacing the old jars), and re-enable anything you disabled.

Common patterns

• Freezes right after a specific mod logs: that mod (or the next to initialise) is deadlocking — disable it.
• Missing or unsupported mandatory dependencies: a required library mod is missing or the wrong version — add/fix it.
• Incompatible mod set! / version mismatch: a mod is built for a different Minecraft or loader version than your pack.
• Crashes only when a player joins, not on boot: that's usually a client/server mod-list mismatch — see Installing a modpack.

Still stuck?

Open a support ticket with the first error from fml-server-latest.log (or a link to the crash report). We can dig into the mod set on the node directly, disable conflicting mods for you, and drop in updated versions so you can see it working.