Errors & Fixes26 articles

Exact error messages, what they mean and the steps that clear them.

Version Downgrade Crash LoopWhy an upgraded world will not open on an older core, and how to get back to the version you were on.3 min read Service Downtime & The 5-Minute RuleMandatory verification periods during micro-outages or scheduled restarts.1 min read «Failed to bind to port»The server cannot claim its own port, usually because something is already holding it.1 min read «Outdated client» or «Outdated server»The version mismatch message, and which side has to change.1 min read «Internal Exception: io.netty…»The generic network disconnect, what the tail of the message means, and how to narrow it.2 min read «java.lang.OutOfMemoryError: Java heap space»The crash that means memory really did run out, and how to tell a leak from a plan that is too small.2 min read «Connection refused: no further information»Nothing is listening at that address and port. Four reasons why, in order of likelihood.1 min read «UnsupportedClassVersionError»: wrong Java versionThe startup crash that means your Java is older than the software you are running.1 min read «Failed to login: Invalid session»The client-side authentication failure, and why restarting the launcher fixes it.1 min read «Can't keep up! Is the server overloaded?»What the number in that message means, and when it is worth worrying about.1 min read «Error: Unable to access jarfile»The server cannot find the file it was told to run, and the three ways that happens.1 min read «Missing dependency» when a plugin or mod loadsSomething you installed needs something else you did not, and the log names it.2 min read The server will not start: a diagnostic orderSix checks that between them explain almost every server stuck on «Starting…».2 min read «Kicked for flying» when nobody is flyingThe anti-cheat that ships with the server, and why lag sets it off.2 min read Corrupted chunks and how much of the world you can saveWhat causes region file damage, what it looks like, and the recovery order.2 min read Everyone disconnects at once: timed outTelling a network problem apart from a frozen server, which look identical from the client.2 min read «Failed to verify username» and authentication failuresWhat the server is checking, why it sometimes cannot, and what not to do about it.2 min read Stuck on «Loading terrain…» foreverThe connection succeeded, so the problem is what happens next.2 min read Mixin errors on a modded serverA crash that names one mod and is usually caused by two.2 min read «Keeping entity that already exists» and duplicate UUIDsA warning that floods the console, and what it says about your world.2 min read «Ticking entity» crashes and how to remove the culpritThe server crashes at the same place every time, and the crash report names what to delete.2 min read «A single server tick took 60.00 seconds»The watchdog killed your server on purpose, and the log above it says why.2 min read Players rejected on join: mod list mismatchModded servers check that the client has the same mods, and the message is not always clear about which.2 min read A player's chat messages do not appearSix causes, in the order that finds it fastest.2 min read «Unknown command» for a plugin you definitely installedThe message is misleading: it usually means «you may not», not «it does not exist».2 min read A permission is set and still does not workFive reasons a node that looks correct does nothing.2 min read