YAML is sensitive to indentation, and a parser error far down the file may come from an earlier misplaced key or list item. Use spaces consistently, then validate a small section around the first reported error with the YAML validator.
Check colons, quoting, nested lists, and values that look like booleans or numbers. Some YAML parsers treat duplicate keys differently, so passing syntax validation may not mean the data has the shape your application expects. Compare the parsed result with the intended structure and run the consumer's own validation.
Keep a working copy before making several indentation changes.
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