Cannot Create Variations Listing: Error - VariationSpecificsSet Container Is Required

[actindo]

I am trying to create a listing with variations.
I created two inventory items with the createOrReplaceInventoryItem call. The result is:

{
  "total": 11,
  "size": 2,
  "href": "/sell/inventory/v1/inventory_item?offset=0&limit=2",
  "next": "/sell/inventory/v1/inventory_item?offset=2&limit=2",
  "limit": 2,
  "inventoryItems": [
    {
      "sku": "muenze-1cent",
      "groupIds": [
        "muenzen"
      ],
      "product": {
        "title": "1 cent Euromünze",
        "aspects": {
          "Nennwert": [
            "1 cent"
          ]
        },
        "description": "Eine stinknormale 1-Centmünze.",
        "imageUrls": [
          "https://ebay-core1-03.dev.actindo.com/ebayImages/61Marketplace_Product/Fotos/muenze-1cent_2018-07-05-171553.jpg"
        ]
      },
      "condition": "USED_EXCELLENT",
      "availability": {
        "pickupAtLocationAvailability": [],
        "shipToLocationAvailability": {
          "quantity": 5
        }
      }
    },
    {
      "sku": "muenze-2cent",
      "groupIds": [
        "muenzen"
      ],
      "product": {
        "title": "Münze 2 Euro cent",
        "aspects": {
          "Nennwert": [
            "2 cent"
          ]
        },
        "description": "Eine stinknormale 2-Centmünze.",
        "imageUrls": [
          "https://ebay-core1-03.dev.actindo.com/ebayImages/71Marketplace_Product/Fotos/muenze-2cent_2018-07-05-171616.jpg"
        ]
      },
      "condition": "USED_EXCELLENT",
      "availability": {
        "pickupAtLocationAvailability": [],
        "shipToLocationAvailability": {
          "quantity": 6
        }
      }
    }
  ]
}

I then created offers for each of the items:

{
  "total": 1,
  "size": 1,
  "href": "/sell/inventory/v1/offer?offset=0&limit=20",
  "limit": 20,
  "offers": [
    {
      "offerId": "20638095018",
      "sku": "muenze-1cent",
      "marketplaceId": "EBAY_DE",
      "format": "FIXED_PRICE",
      "availableQuantity": 5,
      "pricingSummary": {
        "price": {
          "value": "5.0",
          "currency": "EUR"
        }
      },
      "listingPolicies": {
        "paymentPolicyId": "115065654025",
        "returnPolicyId": "112792692025",
        "fulfillmentPolicyId": "115114808025"
      },
      "categoryId": "23204",
      "merchantLocationKey": "actindo_81",
      "tax": {
        "applyTax": false
      },
      "status": "UNPUBLISHED",
      "eBayPlusEligible": false
    }
  ]
}
{
  "total": 1,
  "size": 1,
  "href": "/sell/inventory/v1/offer?offset=0&limit=20",
  "limit": 20,
  "offers": [
    {
      "offerId": "20638096018",
      "sku": "muenze-2cent",
      "marketplaceId": "EBAY_DE",
      "format": "FIXED_PRICE",
      "availableQuantity": 6,
      "pricingSummary": {
        "price": {
          "value": "6.0",
          "currency": "EUR"
        }
      },
      "listingPolicies": {
        "paymentPolicyId": "115065654025",
        "returnPolicyId": "112792692025",
        "fulfillmentPolicyId": "115114808025"
      },
      "categoryId": "23204",
      "merchantLocationKey": "actindo_81",
      "tax": {
        "applyTax": false
      },
      "status": "UNPUBLISHED",
      "eBayPlusEligible": false
    }
  ]
}

I then created the inventary group:

{
  "title": "Euro cent Münzen",
  "description": "Verschiedene normale Euro cent Münzen.",
  "imageUrls": [
    "https://ebay-core1-03.dev.actindo.com/ebayImages/61Marketplace_Product/Fotos/muenze-1cent_2018-07-05-171553.jpg",
    "https://ebay-core1-03.dev.actindo.com/ebayImages/71Marketplace_Product/Fotos/muenze-2cent_2018-07-05-171616.jpg"
  ],
  "variesBy": {
    "specifications": [
      {
        "name": "Nennwert",
        "values": [
          "1 cent",
          "2 cent"
        ]
      }
    ],
    "aspectsImageVariesBy": [
      "Nennwert"
    ]
  },
  "variantSKUs": [
    "muenze-1cent",
    "muenze-2cent"
  ]
}

However, when I am calling publishOfferByInventoryItemGroup, I get the following error:

{
  "errors": [
    {
      "errorId": 25002,
      "domain": "API_INVENTORY",
      "subdomain": "Selling",
      "category": "REQUEST",
      "message": "A user error has occurred. VariationSpecificsSet-Container (Item.Variations.VariationSpecificsSet) ist erforderlich, um einen Artikel mit mehreren Bestandseinheiten einzustellen."
    }
  ]
}

This error probably translates to 'Error - VariationSpecificsSet container (Item.Variations.VariationSpecificsSet) is required to list a Multi-SKU item.'

Item.Variations.VariationSpecificsSet can only be set via the old trading API, which confuses me. Can't I create variations exclusively with the new Inventory API? Or am I missing something else?

I did this in the production environment, and I also made sure the category allows listing variations.

[actindo]

The issue has been solved with the help of the eBay support. The problem was that I did not specify the 'content-language' in all html headers correctly. Apparently I managed to create separate inventory items for de-DE and en-US with the same sku, ond only the en-US items had offers linked. The publishOfferByInventoryGroup call however tried to publish the de-DE offers, which were non-existing.

Summary: Always specify the correct 'content-language'  in the html header (see https://developer.ebay.com/api-docs/static/rest-request-components.html).