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?
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