When activating the .acsm file for a book I bought from the Kobo store (no matching vendor under the eBook Vendor dropdown), I initially chose to activate the book without an ID. Subsequently, I tried to copy the ePub file from my Digital Editions folder to my Sony Reader but I couldn't read it there because it was protected by DRM. I tried de-authorizing and then re-authorizing using an Adobe ID but then I couldn't open the book at all. It says "The vendor account you entered is not associated with the item you are trying to open. Try again." I found another thread where Adobe Staff suggested trying to authorize the book using the original credentials. So I've since tried re-authorizing without ID and it still won't open. I've completely lost access to the book I just bought!
I have exactly the same problem and it's really awful because I forecated to read my book this afternoon!
It seems the book I just bought is lost!!
Try this (it worked for me):
1) In ADEditions, go to Help --> Erase Authorization
2) In ADEditions, go to Help --> Authorize [using Adobe ID]
3) Select Book(s) --> right click --> Remove from Library
4) Re-add book(s) (in case that you have an .acsm file with a link, open this file with ADEditions)