Check out a different library book and transfer it to see if that works.
It is possible that the first book got asssociated with an anonymous/implicit ID that ADE creates if you don't register an explicit one.
If the book is once associated with that ID, you can't use it with a different 'real' ID.
You probably won't even be able to check it back in to the library: you may just have to wait till it expires.
That implicit ID makes the very first use of ADE a little simpler, but it has very limited function and causes lots of problems.
Its main purpose seems to be to lay traps for unwary users.