Common Minecraft Server Errors (and What They Actually Mean)

1 "You need to agree to the EULA"

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

This is the very first thing every new server hits. Minecraft won't start until you accept Mojang's End User License Agreement.

• Open eula.txt in your server folder.
• Change eula=false to eula=true and save.
• Restart. Most panels (including ours) also have a one-click "Accept EULA" toggle.

2 "Failed to bind to port"

**** FAILED TO BIND TO PORT! Perhaps a server is already running on that port?

Something is already using the server's port (default 25565) — usually a previous instance that didn't fully stop, or two servers set to the same port.

• Make sure only one server is using that port. Fully stop the old one first.
• On a panel, your port is assigned — set server-port in server.properties to match what your host gave you.
• If you just restarted, wait a few seconds — the old process may still be releasing the port.

3 "Can't keep up! Is the server overloaded?"

Can't keep up! Is the server overloaded? Running 2000ms or 40 ticks behind, skipping 40 tick(s)

This is lag, not a crash. The server couldn't finish a tick in its 50 ms budget, so it skipped ahead to catch up. An occasional one after a restart or world-gen burst is normal; constant ones mean a real performance problem.

• Profile it — install spark and paste the report into our Spark Report Analyzer to see what's eating ticks.
• Tune your config with the Optimization Wizard and the optimization guide — entities, view/simulation distance, and a heavy plugin are the usual culprits.

4 "OutOfMemoryError: Java heap space"

java.lang.OutOfMemoryError: Java heap space

The server ran out of the RAM it was allowed to use. Two causes:

• Too little allocated — raise the max heap (-Xmx). A modded server on 2 GB will hit this fast; give modpacks 6–12 GB.
• A memory leak — if it climbs to 100% and crashes no matter how much you give it, a plugin/mod or runaway entity build-up is leaking. Use spark's heap summary to find what's holding memory.

5 "Could not reserve enough space for object heap"

Error occurred during initialization of VM. Could not reserve enough space for 8388608KB object heap

Different from #4 — here the server won't even start because you asked Java for more RAM than the machine has (or you're on 32-bit Java).

• Lower -Xmx to below your machine's free RAM (leave headroom for the OS).
• Make sure you're running 64-bit Java — 32-bit caps you around 1.5 GB.

6 "Outdated client!" / "Outdated server!"

Outdated server! I'm still on 1.20.1  |  Outdated client! Please use 1.21

The player's Minecraft version doesn't match the server's. The message tells you which side is behind.

• Easiest fix: everyone uses the exact version the server runs.
• To let multiple client versions join one server, install ViaVersion (and ViaBackwards for older clients) on a Paper/Spigot server.

7 "Internal Exception: io.netty…" / "Connection reset"

Internal Exception: io.netty.handler.codec.DecoderException / Connection reset

A networking-layer error. It's usually one of:

• A version/mod mismatch sending packets the other side can't read (see #6 / #9).
• A flaky connection or a too-aggressive network-compression-threshold — try 256.
• Occasionally a single corrupted packet; a reconnect clears it. If it's constant for everyone, look at versions and plugins that touch packets.

8 "Failed to verify username" / "Authentication servers are down"

Failed to verify username! / Authentication servers are down. Please try again later.

Your server is in online mode and couldn't reach Mojang/Microsoft auth.

• If Mojang's services are actually down, just wait — nothing to fix server-side.
• If you run a cracked/offline server or sit behind a proxy (BungeeCord/Velocity), the backend servers should have online-mode=false and rely on the proxy for auth. Never expose an online-mode=false server directly to the internet — anyone can join as anyone.

9 Mod/plugin mismatch & tick-loop crashes

Mismatched mod channel list / Exception in server tick loop

On modded servers, "mismatched mod channel list" means the client is missing mods the server requires — the player needs the same modpack. A hard "Exception in server tick loop" crash means something broke mid-tick; the cause is in the crash report.

• Open logs/latest.log (or the saved crash-reports/ file) and find the first Caused by: line — it usually names the mod/plugin.
• Update or remove that mod/plugin, then restart. Add one back at a time if you're unsure which it is.

10 Quick recap

1. EULA → set eula=true.
2. Failed to bind → port in use; one server per port.
3. Can't keep up → lag; profile with spark, optimize.
4. OutOfMemoryError → too little RAM or a leak.
5. Can't reserve heap → -Xmx too high / 32-bit Java.
6. Outdated client/server → match versions or use ViaVersion.
7. io.netty / reset → version or network/packet issue.
8. Auth failed → online-mode/auth; never expose offline-mode.
9. Mod mismatch / tick crash → read latest.log, fix the named mod.