Hello there!
This is exactly what happened to me and I eventually found the solution on the Nook site !!
What you need to do is go into the ereader as you would a memory stick/hard drive. Once there you need to delete all ADR files.
When you've done this, you need to restart ADR and plug in your reader.
It will be recognised as new and then it will obtain another ID authorisation.
After this you'll be able to read and transfer books as normal.
I think it must be a bug in either the ADR software or Bookeen...probably the former if forums are anything to go by
Anyway, hope that helps