Thanks Handrawn, you might be right! I turned off transparency and the export worked.

However, I have other Xara pages where I have used the same technique, with transparency enabled, and it has worked. The difference with these pages is that I was exporting a guide region that completely enclosed the object I was exporting, and I needed transparency enabled. This is a useful technique where I want a transparent margin around the object being exported.

So I'm not 100% sure why there is a difference. But you have helped me achieve what I wanted!