Deauthorizing ADE on iMac

I am having the same problem as many others: I can download a book from the library, drag it from Downloads to ADE and read it on my computer.  But suddenly (after many years of doing this) after I move the book to my Kobo and try to open and read it there I get the message saying that it is not authorized.  To solve this Kobo Help has advised that I deauthorize both the Kobo and my computer.  When I try to deauthorize the computer a box pops up asking for a "Vendor Login ID" and a Password.  I spent some time in a Chat with someone at ADE but they could not tell me anything about this.  I do not know who or what the Vendor is and do not know what Password is required - nothing I have tried so far works.  Any ideas?