I got a reply back from Fictionwise when I reported the problem:
Sorry for the error. Due to the high volume of downloads at this time, we have had some server issues. Please try once again to download your titles.
We apologize for any inconvenience this may have caused and thank you for your patience.
I have now been able to download my most recent purchases, and make a start on downloading (again) all my books, just-in-case.