Update: the "remember export path" functionality does *not* work if the source document i saved in same folder as exported png. So there's actually two issues here:
1) export does not work if the svg document isn't saved
2) if export is attempted from a rectangle to a png placed "beside" the svg, the path is not remembered.
Workaround for (1) is to save document anywhere.
Workaround for (2) is to export png's to some other place than the .svg.
Update: the "remember export path" functionality does *not* work if the source document i saved in same folder as exported png. So there's actually two issues here:
1) export does not work if the svg document isn't saved
2) if export is attempted from a rectangle to a png placed "beside" the svg, the path is not remembered.
Workaround for (1) is to save document anywhere.
Workaround for (2) is to export png's to some other place than the .svg.