Creating Requests on a SD cloud instance return 400 (despite formerly working parameters)

I post to rest endpoint rest/servicedeskapi/request with body set to

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

And I get 400

{
“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”: [
“”
]
}
}

I can’t see what’s missing, as this request worked before.

What could be missing?