This can happen because either Xara doesn't have enough permissions to write to the directory you are trying to export to, or there is something prevent Xara from doing so.
1) Try closing the program, right-click the program shortcut > Run as Administrator. See if you can export now.
2) If you still can't, are you running Avast anti-virus security software? If so, this is currently known to prevent Xara from export correct. Disable it and it should work again.