RLCraft has a reputation for being crash-prone, but almost every common crash has a known cause. Here are the ones we see most often when hosting RLCraft servers.
"Unsupported class file major version" on startup
This means you're running the wrong Java version. RLCraft on Minecraft 1.12.2 requires Java 8. Modern control panels often default to Java 17 or 21 for new servers, which will crash RLCraft immediately on boot. Check your server's Java runtime setting and switch it to Java 8 specifically.
Server freezes or crashes during world generation
RLCraft's terrain, including Lycanites dungeons, Battle Towers, and Ice and Fire dragon lairs, is expensive to generate on the fly. If players spread out and explore too much new terrain at once, the server can hit an out-of-memory error or a long tick freeze that looks like a crash.
Fix: Pre-generate the world with a chunk pre-generation tool before opening the server to players. A 3000-block radius covers most early-to-mid game exploration.
Crash after adding a new mod
RLCraft is built around exact mod versions that are tested to work together. Adding or updating even one mod, including seemingly unrelated ones, can trigger a mod loading conflict on startup. If you added something and now the server won't boot, remove it first and confirm the base modpack still starts before troubleshooting further.
"Lycanites Mobs" related crash logs
Lycanites Mobs is the single most crash-prone component of RLCraft because of how much it spawns. If your crash log references lycanitesmobs, try:
- Lowering
mobSpawnChanceanddungeonMobRangeinconfig/lycanitesmobs/general.cfg - Reducing view distance to 6-8
- Increasing
max-entity-crammingslightly if the crash is cramming-related
Out of memory errors with "enough" RAM allocated
If you're allocating what looks like plenty of RAM but still hitting OOM crashes, check whether your -Xmx and -Xms values are actually being applied by your host's startup script. Some panels silently cap memory below what's configured. See our RLCraft RAM requirements guide for the correct baseline numbers by player count.
Corrupted world after a crash
Always run frequent automated backups. RLCraft's custom structures make manual world repair difficult, and a corrupted region file after a crash is much easier to fix by rolling back a recent backup than trying to patch it manually.
For the full setup and difficulty tuning walkthrough, see the RLCraft server setup guide.