Error creating XPS file

Error creating XPS file: Source _rels/.rels not found in archive

If you see the above error when trying to use xpstopng, xpstopdf, xpsto* tools provided by libgxps, then you probably suffer from this bug:
https://bugzilla.gnome.org/show_bug.cgi?id=670895

In short, the problem lies with libarchive versions prior to 3.x and zip files created by windows.
Solution?
a) Just update to 3.x or newer and you'll be good to go.
b) As a workaround, you can recreate the zip file:

unzip lorem-ipsum-and-image.xps -d temp/
cd temp/
zip ../newfile.xps *
cd ..

You now have a working copy of your XPS document.