I cannot create a SD request via API (any more) using POST rest/servicedeskapi/request

I do a POST to {{cloudBaseUrl}}/rest/servicedeskapi/request with the body

{
“requestFieldValues”: {
“summary”: “Soll das hier übersetzt werden?”,
“description”: “Ja, bitte!”,
“attachment”: []
},
“raiseOnBehalfOf”: “557058:c755c582-8f73-4432-a3e1-ffd318d0df1c”,
“requestParticipants”: [],
“serviceDeskId”: “59”,
“requestTypeId”: “783”
}

and I get:

{
“errorMessage”: "Your request could not be created. Please check the fields have been correctly filled in. ",
“i18nErrorMessage”: {
“i18nKey”: “sd.validation.request.creation.failure.required.field”,
“parameters”: [
“”
]
}
}

When I call endpoint GET /rest/servicedeskapi/servicedesk/59/requesttype/783/field is get

{
“requestTypeFields”: [
{
“fieldId”: “summary”,
“name”: “Summary”,
“description”: “”,
“required”: true,
“defaultValues”: [],
“validValues”: [],
“jiraSchema”: {
“type”: “string”,
“system”: “summary”
}
},
{
“fieldId”: “description”,
“name”: “Description”,
“description”: “”,
“required”: false,
“defaultValues”: [],
“validValues”: [],
“jiraSchema”: {
“type”: “string”,
“system”: “description”
}
},
{
“fieldId”: “attachment”,
“name”: “Attachment”,
“description”: “”,
“required”: false,
“defaultValues”: [],
“validValues”: [],
“jiraSchema”: {
“type”: “array”,
“items”: “attachment”,
“system”: “attachment”
}
}
],
“canRaiseOnBehalfOf”: true,
“canAddRequestParticipants”: true
}

So I need the three properties: summary, description and attachment.
When I omit the attachment property in the initial request, same result. When I put in some other value like [{}] oder “string” the I get a

{
“errorMessages”: [
“Internal server error”
],
“errors”: {}
}

What do I have to set in, if I have no attachment?