Here I am provideing you the complete scenario where I am getting the error while posting request for creating a new account.
I am using Intuit OAuth Access and OAuth API console for testing (Using all security tokens).(https://appcenter.intuit.com/Playground/OAuth)
And the URL and request:
URL: https://services.intuit.com/sb/account/v2/570357960
Method : Post
Format: XML/JSON
Request:
<?xml version="1.0" encoding="UTF-16"?><Add xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" RequestId="4df87bw2-916a-
4r95-h5d6-06dce3667562"
xmlns="http://www.intuit.com/sb/cdm/v2"><ExternalRealmId>570357960</ExternalRealmId><Objectxsi:type="Account"><Name>CurrentAccount</Name><Active>true</Active><Type>Expense<
/Type><Subtype>Expense</Subtype><AcctNum>1111</AcctNum></Object></Add>
And getting the response (error in html):
<div class="content-container"><fieldset>
<h2>500 - Internal server error.</h2>
<h3>There is a problem with the resource you are looking for, and it cannot be displayed.</h3>
and I have also checked with RESTClient utility, but getting the same above error.
Please note that the same url (https://services.intuit.com/sb/account/v2/570357960) with same security tokens is working fine for GET data , I am getting data from QB desktop into my online Application.