[DiapDealer]

Has anything changed with regard to your computer's hardware recently? Did you upgrade the software or did you uninstall (saving your library) and then re-install/re-register? Or install the upgrade under a new user account (or the same account with different admin rights)?

Usually upgrading (technically installing over the old version) doesn't cause the problem you're having. But I have seen something similar happen in the above situations I mentioned. They can result in different hardware/user-based tokens being generated for the software. Which means that any books that were downloaded using previous tokens won't be readable by the new version of the software. Hence the reason for re-downloading books that are based on the new security/DRM tokens.