I have used ADE for a number of years on my desktop computer. I use it with a Kobo reader which has been authorized with ADE. I bought a new computer, downloaded ADE, signed in with my ID (which is all lower case), and it authorized me using all Upper Case.This will not work with my existing ebooks on my reader as there is a mismatch. It will also not work with new books. I need to have one Adobe account, with my email address all in lower case letters, before I can read Adobe DRM protected books again.
FWIW, I have a very common name. And that name is my email address on gmail.com. On a number of occasions some other person with the same name has registered for various things using my address in error (or confusion). I believe this may haver happened here, with the difference between the two accounts being the capitalization.