Mike,
Where is the error generated? In the Export to XML action or in the REST action?
We always set a custom error handler on the Call REST action. There we create a log where we store the details.
As you can see is that the Export to XML will also in our case produces a string that we put in the log. This way you capture the errors generated and have the XML you are trying to POST.
Does this help?
Regards,
Ronald
[EDIT]
Hmm, is it me or are the picture insert no longer working. I can see the picture only when I am in edit mode.
This is the URL to the picture https://1drv.ms/i/s!At74fUE_gLfwgs8baHki_PbDbzI9JA