"You may need an additional loader to handle the result of these loaders. "

[CrossedCarpet]

Solved by changing package.json:

  "browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
    "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
    ]
  }

To:

  "browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
        "development": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ]
  }

Maybe it's worth updating the repo with it.

[limikael]

although... I had to change both production and development to:

  "browserslist": {
    "production": [
      "chrome >= 67",
      "edge >= 79",
      "firefox >= 68",
      "opera >= 54",
      "safari >= 14"    ],
    "development": [
      "chrome >= 67",
      "edge >= 79",
      "firefox >= 68",
      "opera >= 54",
      "safari >= 14"
    ]
  }

Otherwise I got an error Cannot convert a BigInt value to a number. I found the fix here: starknet-io/starknet.js#37

Worked for me, but can't explain the actual root cause though...