I try to create a work package via APIv3. I have configured a simple tracker: only a custom field and the subject. The custom field is required. Field definition from schemas request:

"customField4":    {
      "type": "String",
      "name": "Instanz",
      "required": true,
      "hasDefault": false,
      "writable": true,
      "visibility": "default",
      "maxLength": 1024
   },

The content of the post request is:

{
"customField4":"Test",
"subject":"Test"
}

I get allways this response:

{"_type":"Error","errorIdentifier":"urn:openproject-org:api:v3:errors:PropertyConstraintViolation","message":"Instanz muss ausgefüllt werden.","_embedded":{"details":{"attribute":"customField4"}}}