Server won't start

Status pill stuck on grey, console refuses to scroll past the JVM banner, or you see Exception in server tick loop and nothing else? Don't reinstall yet — 9 times out of 10 the console already names the culprit. This guide walks the diagnosis, ordered by how common each cause is.

Step zero: read the console

Before doing anything, scroll the console up to the first error or stack trace. Minecraft's logs cascade — a single root cause produces dozens of follow-up errors, and the bottom of the log is almost never the real problem. Look for lines that start with:

• [ERROR] followed by a JVM exception name (OutOfMemoryError, ClassNotFoundException, etc.)
• Caused by: — the chain almost always points at the broken file.
• Plugin or mod names in brackets right before the trace.

Out of memory

The single most common failure mode for modded servers. The console will contain one of:

java.lang.OutOfMemoryError: Java heap space
java.lang.OutOfMemoryError: GC overhead limit exceeded
Could not reserve enough space for object heap

Your plan doesn't have enough RAM for what's running on it. Two fixes:

• Upgrade your plan. Open the Billing Panel → pick the server → Upgrade/Downgrade. See Upgrading or downgrading for the full flow.
• Slim down what's running. Big-ass modpacks (ATM, FTB Stoneblock) need 6–10 GB minimum. Anti-cheat + claims + economy + world-edit stack on Paper eats RAM too. Audit your /mods and /plugins for things you don't actually use.

Missing dependency

A mod or plugin you installed needs something else that isn't there. Look for:

[ERROR] Could not load 'plugins/SomePlugin.jar'
Caused by: java.lang.NoClassDefFoundError: net/milkbowl/vault/economy/Economy
ModLoadingException: Mod jei requires fabric-api 0.91.0 or higher

The named class (or mod) lives in another JAR you haven't installed. Common offenders:

• Vault — required by most economy / permission plugins.
• ProtocolLib — required by many anti-cheats and combat plugins.
• PlaceholderAPI — required by chat/scoreboard plugins.
• Fabric API / NeoForge — required by Fabric/NeoForge mods.
• Cloth Config, Architectury, Connector — shared mod libraries.

Install the missing dependency via Adding plugins or Adding mods, then restart.

Version mismatch (loader / MC)

A JAR is built for the wrong Minecraft or wrong loader version. Symptoms:

java.lang.UnsupportedClassVersionError: SomeMod has been compiled by a more recent version of the Java Runtime
Plugin requires server version 1.20+ but server is running 1.19.4
Mod 'examplemod' (ID: examplemod) requires Forge 47.x, but found 50.x

Fixes:

• Re-download the JAR for the exact MC + loader version your server runs. version in the console tells you both.
• Or update the server side — see Changing version.

Duplicate JARs

Two copies of the same plugin or mod in the folder. Startup crashes immediately with a class-loading error. Easy fix:

1. Open Files → /plugins (or /mods).
2. Sort by name. If you see SomeMod-1.0.jar and SomeMod-2.0.jar, that's the problem.
3. Delete the older one. Restart.

World corruption

Less common but more dramatic. You'll see:

Failed to load level: Could not read level.dat
EOFException at NBTReader
Error reading region file world/region/r.0.0.mca

Almost always caused by an unclean shutdown (server killed mid-save, host node power loss). Restore the latest Backup via the Backups tab — most cases resolve in seconds. If you don't have a backup, MCA file editors like MCASelector can sometimes salvage the corrupted chunks.

Disk full

Less obvious — server boots part-way then bails with:

• java.io.IOException: No space left on device
• Failed to save level.dat
• Cannot create temp file

Check disk usage on the dashboard (it's one of the four resource graphs). If it's at 100%, the usual culprits:

• Old backups — clean them up on the Backups tab.
• Bloated logs at /logs — delete anything older than a few days.
• Multiple old worlds in /worlds or /world_old.
• Modpack updates left old /mods JARs behind.

Still tight after cleaning? Upgrade to a plan with more disk.

EULA not accepted (vanilla)

First-boot vanilla servers refuse to start until you sign Mojang's EULA. The console says:

You need to agree to the EULA in order to run the server.
Go to eula.txt for more info.

Open Files → eula.txt → change eula=false to eula=true, save, restart. PulsarServers presents this in the install wizard but if you reinstalled manually, you may have to redo it.

Port conflict

Rare but possible after migrations:

java.net.BindException: Address already in use
FAILED TO BIND TO PORT! Perhaps a server is already running on that port?

Edit server.properties and confirm server-port matches the port your panel allocated (visible on the dashboard). Don't pick a random number — the panel firewall only opens specific allocations.

When all else fails

If you've tried everything, the dashboard's Settings → Reinstall button wipes the server clean and re-runs the install script. It's destructive — download a backup first — but it fixes anything caused by accumulated junk. Same egg, same plan, same IP.

Still stuck after trying everything? Open a ticket with the full boot log and we'll dig in.