Disconnect your device from the USB port
Now restart Adobe Digital Edition and using CTRL-SHIFT-D to deauthorize the computer and select Erase Authorization.
Now go back to Adobe Digital Edition, Help and Authorize Computer
Now reconnect the device to the USB port
Now select your device in the Adobe Digial Edition and Authorize the device
Now get a library book in the Adobe Digital Edition and then add it to the Device
Hopefully things will work fine.
I hope it helps.