Forge Server Tools · every failure mode · guides
A config file is invalid — ERROR
A .toml/.json config is malformed — a hand-edit that broke the syntax, or a file truncated by a crash mid-write.
Delete the named config file and let the mod regenerate a default, then re-apply your changes a few at a time. Keep a backup before hand-editing configs. A truncated file after a crash looks identical to a typo.
Search your crash report or latest.log for any of these:
ConfigLoadingException Failed to load config file com.electronwill.nightconfig.core.io.ParsingException
The free edition covers the six most common startup failures, and this is not one of them — it is in the paid tool, which checks all 24 and names the mod jar from the stack trace. The fix above is the same one it prints, so you do not need it to solve this.