This is my problem. I installed version 4.5 of adobe digital editions and my older nook would not work with the newer version. nook support had me uninstall 4.5 and install 3.0. now when I try to read an epub downloaded book I am getting an authorization error
Please refer the troubleshooting steps provided in this help document: Error messages and troubleshooting tips for activation errors in Adobe Digital Editions
Check if it helps.