In my case, that was an email that was from a domain Apple did not like. When I switched to a more "common" account (something gmail related) it accepted my users.
zebishop
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I'm encountering the exact same issue. No error in the UI, error 500 with a json object :
{
"validationResults": [
{
"field": null,
"localizedField": null,
"validationRuleName": null,
"messageKey": null,
"error": null,
"propertyName": null,
"errorShortMessage": null,
"errorFlag": null,
"errorString": "An unknown error has occurred.",
"errorNumber": -21607
}
],
"status": null,
"code": null
}
https://appstoreconnect.apple.com/sandbox/v2/account/create