GetAllCultures Failes with 'Culture is not supported.'

Questions
1

Currently we struggle with the following API call which is performed by Umbraco:
umbraco/backoffice/umbracoapi/language/GetAllCultures

The response of the call is:

{
    "ExceptionMessage": "Culture is not supported. (Parameter 'name')\r\naa is an invalid culture identifier.",
    "ExceptionType": null,
    "StackTrace": null
}

This call should return all available cultures like:

{
    "aa": "Afar",
    "aa-DJ": "Afar (Djibouti)",
    "aa-ER": "Afar (Eritrea)",
    "aa-ET": "Afar (Ethiopia)",
    "af": "Afrikaans",
    "af-NA": "Afrikaans (Namibia)",
    "af-ZA": "Afrikaans (South Africa)",
    "agq": "Aghem",
    "agq-CM": "Aghem (Cameroon)",
    "ak": "Akan",
    "ak-GH": "Akan (Ghana)",
    Etc.....
}

As far as I can see in the controller this code should be executed and nothing more right?:

[HttpGet]
public IDictionary<string, string> GetAllCultures()
    => CultureInfo.GetCultures(CultureTypes.AllCultures).DistinctBy(x => x.Name).OrderBy(x => x.EnglishName).ToDictionary(x => x.Name, x => x.EnglishName);

How can this controller endpoint ever fail?

Because it cannot retrieve any culture, we also cannot add any new ones via Umbraco. Currently we have one culture configured in the DB and it looks like this:

"id","languageISOCode","languageCultureName","isDefaultVariantLang","mandatory","fallbackLanguageId"
1,en-US,English (United States),0,0,
This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/112315-getallcultures-failes-with-culture-is-not-supported