question

I'm trying to make a demonstration of my implemented API-functionality, preferably using the sandbox environment, but this requires to have a seller accout with orders. For this I'm trying to insert an order with the createOrReplaceInventoryItem call but I'm only getting this error with a 500 response:

{"errors": [{
    "errorId": 2003,
    "domain": "ACCESS",
    "category": "APPLICATION",
    "message": "Internal error",
    "longMessage": "There was a problem with an eBay internal system or process. Contact eBay developer support for assistance",
    "parameters": [{
        "name": "reason",
        "value": "Failed to transform underlying error response, see logs."
    }]
}]}

I've registered the sandbox user and was able to get an access token, but this doesn't work. The authorization token is included in the http header. I've been trying the whole day with different variations. Any Ideas?

http.setRequestProperty("Authorization", "Bearer " + token.getAccessToken());
http.setRequestProperty("Content-Language", "en-US");
http.setRequestMethod(method == null ? "GET" : method);
http.setDoOutput(true);
http.setDoInput(true);

if (content != null)
http.setRequestProperty("Content-Type", "application/json; utf-8");

http.connect();
System.out.println(method+ "|"+http.getRequestMethod());

if (content != null) {

try (OutputStream os = http.getOutputStream()) {
byte[] input = content.getBytes("utf-8");
os.write(input, 0, input.length);
os.close();
}


} else {
}