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.
Or skip the reading entirely
Paste your crash-report.txt into the Crash Doctor and you'll get the most likely cause plus the one thing to try first. Free, no signup, secrets auto-stripped from input.