Downloaded books greyed out, even after 4.5.11, re-authorisation and everything I can think of!

I've been using ADE and Sony Reader software with my Sony PRS-650 Reader for 11 years to download books from our public library. All of a sudden, downloaded books are greyed out in the Reader software on my PC and won't download to the device itself. I've read all the relevant stuff on here, have upgraded to ADE 4.5.11, unauthorised and re-authorised both my PC and my reader, rebooted everything I can think of but the problem remains. I've tried deleting the books and the downloads. Each time I download, the book is just greyed out. If I redownload, the software says the book is already on the device.  Really frustrating - any ideas are welcome!