Minecraft server crash fixes
One guide per error pattern. The first line of each tells you what to try; the rest tells you why. If you want to skip the reading and paste your crash-report.txt instead, the Crash Doctor does it automatically.
java.lang.UnsupportedClassVersionError
Fix the Minecraft server UnsupportedClassVersionError crash — the Java runtime is newer or older than the mod was compiled for. One-line fix below.
java.lang.NoSuchMethodError
Fix Minecraft NoSuchMethodError crashes — almost always a mod-version mismatch between two mods that disagree on an API method.
java.lang.OutOfMemoryError: Java heap space
Fix Minecraft server OutOfMemoryError crashes — the JVM ran out of allocated heap. Most modpacks need 8–16 GB; here's how to size correctly.
Mixin apply failed (MixinTransformerError)
Fix Minecraft Mixin apply failed crashes — one mod tried to inject into a class another mod or MC version changed. Usually update or disable the offending mod.
Sinytra Connector ERROR
Fix Sinytra Connector errors on Minecraft Forge servers — almost always a Fabric mod that doesn't translate cleanly. Disable or replace it.
No space left on device (Minecraft IOException)
Fix the Minecraft server 'No space left on device' crash — the container ran out of disk. Find what's eating space + how to recover.
Permission denied / AccessDeniedException
Fix Minecraft server 'Permission denied' crashes — the Java process can't write a file. Usually wrong file ownership after an upload or restore.
java.lang.StackOverflowError
Fix Minecraft StackOverflowError crashes — a method called itself infinitely until the JVM thread stack ran out. Almost always a mod bug.
Bad config file (TOML / JSON parse error)
Fix Minecraft mod config parse errors — a config/*.toml or *.json got corrupted, manually edited wrong, or out-of-spec after a mod update. Restore or regenerate.
Registry / world corruption (unregistered block or entity)
Fix Minecraft 'Encountered an unregistered' and 'ResourceLocation X not registered' crashes — the world references content from a mod that's no longer loaded.
Failed to save world / Failed to save chunk
Fix Minecraft 'Failed to save world' and 'Failed to save chunk' errors — Minecraft couldn't flush data to disk. Usually disk full, permissions, or corruption.
Watchdog tick timeout (single server tick took)
Fix Minecraft server watchdog crashes — a tick took longer than 60s and the server killed itself. Usually a heavy mod, not a real bug.
Missing mod dependency (mod requires X which is not present)
Fix Minecraft 'mod requires X which is not present' errors — the loader found a mod but its required companion mod isn't installed. Add it.
java.lang.NoClassDefFoundError
Fix Minecraft NoClassDefFoundError — a class that was available at compile time is missing at runtime. Almost always a missing or mismatched mod.
EADDRINUSE — Address already in use (port 25565)
Fix Minecraft 'Address already in use' — another process is already bound to port 25565. Kill the old process or change the port.
Ticking entity crash (java.lang.NullPointerException in entity tick)
Fix the Minecraft 'ticking entity' crash — a specific entity in your world is throwing an exception every tick. Find its UUID and remove it.
Chunk corruption (ChunkSerializationException / DFU migration failed)
Fix Minecraft chunk corruption errors — one or more region files are unreadable. Delete the corrupt region file or restore from backup.
License key was not accepted (FiveM / FXServer)
Fix the FiveM 'license key was not accepted' error — your sv_licenseKey in server.cfg is invalid, expired, or doesn't match the IP. Re-generate it at keymaster.fivem.net.
Resource dependency failed to start (FiveM)
Fix FiveM 'resource X has a dependency on Y which failed to start' — install the missing dependency and ensure it's started before the dependent resource.
oxmysql / mysql-async: Database connection failed (FiveM)
Fix FiveM oxmysql or mysql-async 'database connection failed' errors — wrong connection string, MySQL not running, or firewall blocking the connection.
OneSync not enabled / entity limit exceeded (FiveM)
Fix FiveM OneSync errors — add 'onesync on' (or 'onesync legacy') to server.cfg and ensure your license key tier supports it.
FXServer crash loop (server keeps restarting)
Fix a FiveM FXServer crash loop — the server restarts immediately after starting. Read the last log, find the crashing resource, disable it and restart.
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