I think I encountered that once and I think I reinstalled the eBookwise driver, which solved the problem. Sorry I can't remember but try that and let me know. I will check this thread every day now, since they only seem to send out updates once a week.
Another problem you might encounter is when you first try to upload the books to your reader, you will get an error. You then need to reset the reader and try again and so far it has always worked for me.