Troubleshooting modpack errors: fix crashes, missing mods, and lag

Troubleshooting modpack errors: fix crashes, missing mods, and lag

Modpacks bring huge fun—and occasionally confusing errors. This guide is your calm checklist for solving the most common problems so your HostChicken server and clients can get back to playing.

Rule #1: Read the first error, not the last

Logs look noisy. Scroll up to find the earliest “Caused by” entry or the first red stacktrace—it usually names the mod or mismatch causing the crash.

Startup crashes

Symptoms: Game closes on launch, red error screen, long stacktraces.

Checklist:

• Wrong loader: Confirm Forge vs Fabric and the exact version (e.g., Fabric 0.15.x for MC 1.20.1).

• Missing dependency: The log names it. Install the exact version.

• Wrong mod version: Use the mod built for your pack’s Minecraft version.

• Client‑only mod on server: Remove maps, shaders, or UI mods from the server `mods/`.

• Corrupted download: Re‑download the mod or pack.

Join errors: “Missing mods” or “Incompatible mod set”

• Your client must have the same mod list as the server (except server‑only mods).

• Use the server’s provided mod list or pack manifest.

• With Prism or CurseForge, ensure the instance version matches the server.

Performance issues: low TPS or stuttering

• Lower server view distance to 8–10 and simulation to 5–6.

• Use Paper (for plugin servers) or recommended performance mods for Fabric/Forge.

• Trim laggy farms: slow hoppers, reduce entity counts, avoid always‑on redstone clocks.

• Pre‑generate chunks for exploration‑heavy sessions.

Out of memory errors

• Increase RAM within reason (light packs: 2–4 GB, medium: 4–6 GB, heavy: 6–8 GB+).

• Don’t over‑allocate; it can cause GC stalls.

• Close background apps. On servers, schedule restarts to clear bloat.

Update breakage

• Always back up before updates.

• Stage updates on a clone and test for 15–30 minutes.

• Read change logs; some updates break worlds or require config changes.

World corruption or odd behavior

• Restore from the last good backup.

• Test the problematic mod on a clone to confirm.

• Keep a “golden” backup for major milestones.

Fabric/Forge specific tips

• Fabric mixin errors: Update Fabric loader and the implicated mod.

• Forge classloading errors: Ensure matching Forge build; avoid mixing minor MC versions.

Tooling that helps

• Prism Launcher: Clean instance management, easy imports for `.mrpack` and CF zips.

• JEI/REI: Recipe viewers that help confirm mods loaded correctly.

• Spark/Paper timings: Performance profiling (server‑side).

Ask better questions (when you need help)

Share:

• Minecraft + loader versions (e.g., 1.20.1 Fabric 0.15.x)

• The pack name and version

• Exact error snippet (first “Caused by” lines)

• What changed right before it broke

A calm recovery plan

1. Stop the server; take a fresh backup of the broken state (for forensics).

2. Restore the last known good backup.

3. Re‑apply changes one at a time on a clone until you find the culprit.

4. Fix or drop the problematic mod and communicate the outcome.

With a steady checklist and backups, most modpack problems are fixed in minutes. Keep your notes, use staging servers, and you’ll spend more time adventuring and less time debugging.