It took me a while to track this down. If you're saving any data that contains a LabVIEW icon, and that icon contains more than 1 layer, XML can't handle it. See attachment.
"If you weren't supposed to push it, it wouldn't be a button."
" Although you can flatten and unflatten LabVIEW variant data to and from XML, LabVIEW returns an error if the variant data is an element of a cluster or is an array of variants."
