RESTlet Http calls fails with 500 UNEXPECTED_ERROR

Greetings everyone!
I have a problem that seems to have started since Version 2018 Release 2.

We perform two rest calls. The first is a GET on...
https://rest.netsuite.com/rest/roles
...where we get the POST Customer endpoint

"restDomain": "https://9876543-sb1.restlets.api.netsuite.com"

where "9876543" is our account id.

Then we use that restDomain endpoint to perform a Post api call to insert a new customer. The raw body call is like
{
  "recordtype": "customer",
  "subsidiary": "1",
  "parent": null,
  "title": "-",
  "url": null,
  "companyname": "My Company",
  "entityId": "Customer Name",
  "phone": "2345123456",
  "fax": "",
  "startDate": null,
  "email": "customer@test.gr",
  "category": "1",
  "currency": "1",
  "addresses": [
    {
      "addressBook": {