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.
I'm not sure how ADE usually handles capitalization: I have certainly managed with an all lowercase authorization to both ADE 1.7.2 and to ADE 2.0.
Here are a couple of things to try.
Sometimes ADE gets its registration/activation confused and in a semi-authorized state.
Unfortunately, it often then gives misleading error messages about what is wrong.
A common incorrect message informs you that the ID is already in use on another computer and cannot be reused.
This can often be resolved by completely removing any authorization using ctrl-shift-D to the Library screen on ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize with your (old) Adobe ID.
There are lots of bugs in ADE2.0. Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172