RLCraft has a reputation for being crash-prone, but almost every common crash has a known cause. Here are the most common ones and how to fix them.
"AppClassLoader cannot be cast to class java.net.URLClassLoader" on startup
This means the server is running on a Java version newer than 8. RLCraft on Minecraft 1.12.2 requires Java 8. Forge for 1.12.2 starts through Mojang's old LaunchWrapper, which expects the Java 8 class loader, so on Java 9 or newer the server stops right away: the log shows "A problem occurred running the Server launcher" and a java.lang.ClassCastException with the message above. Modern control panels often default to Java 17 or 21 for new servers. 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. RLCraft's own server notes ask for this once more than 10 players will play, with the Chunk Pregenerator mod and a 10,000 block radius.
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
"Weight Scale"(0.5 halves Lycanites spawns) or"Mob Type Limit"inconfig/lycanitesmobs/spawning.cfg - Reducing view distance to 8, or 6 at the lowest
- Raising the
maxEntityCramminggame rule slightly 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 baseline numbers, or the RLCraft server hosting page for the plan sizes we suggest.
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.