Fabric Loader Crash: IllegalArgumentException: can't parse argument number: 4

What the log looks like

(?:java\.lang\.)?IllegalArgumentException: can't parse argument number: 4
at net\.fabricmc\.loader\.impl\.util\.Localization\.format\(Localization\.java:\d+\)

How to fix it

1. Open `.minecraft/logs/latest.log` in any text editor and look right above `Uncaught exception in thread "main"` for `[main/WARN]: Mod resolution failed`.
2. Check the `Reason:` line to see which mods conflict (`NEG_HARD_DEP ... {breaks ...}`) or require a different version (`HARD_DEP ... {depends ...}`).
3. Remove or update the conflicting mods listed in that line—for example, do not install OptiFabric/OptiFine alongside Sodium and Iris, and update Iris or MaLiLib if one lists an older version under `breaks`.
4. Update Fabric Loader to the latest version (or add `-Duser.language=en` to your launcher JVM arguments) so Fabric can display normal mod error popups in the future.

Check your own log