As Christine says, try to "Create bitmap copy" near the bottom of the Arrange Menu (shift+Ctrl+C) within Xtreme itself, rather than truly exporting it. Once complete then export from Xara - it should work.

I often do things like apply 2 transparency filters on a given object by applying one linear filter (for example) then group the object by itself to apply a fractal cloud transparency filter (for example). Sometimes trying to export that as a transparent PNG (for example) causes your problem. But creating a PNG within Xara seems to work.

GP