This has been suggested for a similar issue with other readers. It seems to be a situation where the authorization get confused, and even removing it doesn't really remove it.
- deauthorize the ereader (ctrl-shift-E to ADE while device plugged in)
- close ADE
- Look at the ereader device showing up as a drive on a file manager such as Windows Explorer
Make sure you are showing hidden files. - You will see file or directory with a name like '.adobe-digital-editions'
- Delete it.
- restart ADE
- try to reauthorize the device.
Horrid. Good luck.