Just found that there's a change in the user creation parameters between 19.8 and 19.12 - two additional (mandatory?) parameters were added:

{

  "authSystem": "string",

  "authority": "string",

  "domain": "string",

  "internalAccount": true,

  "password": "string",

  "passwordExpiraryDate": 0,

  "passwordExpireDays": 0,

  "readOnly": true,

  "rootUser": true,

  "userName": "string"

}