Invalid polygon inside a MultiPolygon

What the Information System does

A MultiPolygon is a set of polygons (GeoJSON File Description, Definitions), and each of them has to be valid: the problems on this page's related rules apply to every part. One invalid part is enough for the geometry to be rejected (Information System validation rules, Geolocation invalid geometry).

A broken example

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {
        "ProducerCountry": "CI",
        "ProductionPlace": "One bad block",
        "Area": 2.4
      },
      "geometry": {
        "type": "MultiPolygon",
        "coordinates": [
          [
            [
              [-5.123456, 6.123456],
              [-5.122456, 6.123456],
              [-5.122456, 6.124456],
              [-5.123456, 6.124456],
              [-5.123456, 6.123456]
            ]
          ],
          [
            [
              [-5.113456, 6.123456],
              [-5.112456, 6.123456],
              [-5.112456, 6.124456],
              [-5.113456, 6.124456]
            ]
          ]
        ]
      }
    }
  ]
}

What the checker reports for this file:

1. One of the 2 shapes in this MultiPolygon is invalid, which makes the whole feature invalid: the valid shapes are rejected with it., line 13, column 24

Fix it in EUDRplot

1. Open the file in the checker. Alongside this error, the checker reports the underlying problem (an open ring, a crossing, a hole) naming which of the shapes in the MultiPolygon it is in.
2. Fix that problem. Apply the fix offered for the underlying problem. Once that shape is valid, this message goes away with it.

Fix it in QGIS

1. Separate the parts. Run Multipart to singleparts from the Processing Toolbox, then Check validity to find the bad part.
2. Fix and recombine. Fix it as you would any polygon, then run Collect geometries on the producer's field if you need a single MultiPolygon again.