Cheers @bitprecision, I had a look into this with @Jon earlier today and we were able to confirm the problem. I have added this to our internal bug tracker and we’ll of course confirm once this is fixed.
We will also be releasing an update for the HTTP Request node very soon (as described here by @Nivb_6) which will let you use the Quickbooks credentials for any request. This should allow you to make the correct API request manually as a workaround until this problem is properly fixed in the Quickbooks node.